[compiler-rt] Fix detecting _Float16 support for secondary targets (#117813)

[etcwilde]

Cherry-picking from llvm#117813 and llvm#133952

It turns out we were not passing -m32 to the check_c_source_compiles() invocation since CMAKE_REQUIRE_FLAGS needs to be string separated list and we were passing a ;-separated CMake list which appears to be parsed by CMake as 'ignore all arguments beyond the first'. Fix this by transforming the list to a command line first.

With this change, Clang 17 no longer claims to support _Float16 for i386.

And to pass all of the required flags:

The try-compile mechanism requires that CMAKE_REQUIRED_FLAGS is a
space-separated string instead of a list of flags. The original code
expanded BUILTIN_FLAGS into CMAKE_REQUIRED_FLAGS as a
space-separated string and then would overwrite CMAKE_REQUIRED_FLAGS
with TARGET_${arch}_CFLAGS prepended to the unexpanded
BUILTIN_CFLAGS_${arch}. This resulted in the first two arguments being
passed into the try-compile invocation, but dropping the other arguments
listed in BUILTIN_CFLAGS_${arch}.

This patch appends TARGET_${arch}_CFLAGS and BUILTIN_CFLAGS_${arch} to
CMAKE_REQUIRED_FLAGS before expanding CMAKE_REQUIRED_FLAGS as a
space-separated string. This passes any pre-set required flags, in addition to
all of the builtin and target flags to the Float16 detection.

(cherry picked from commit a4c8ef0 and commit 0d3f5ec)

[etcwilde]

@swift-ci please test

[etcwilde]

macOS failure:

Resolving deltas:  49% (3369949/6877446)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandIn(CliGitAPIImpl.java:2846)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl.launchCommandWithCredentials(CliGitAPIImpl.java:2185)
	at org.jenkinsci.plugins.gitclient.CliGitAPIImpl$1.execute(CliGitAPIImpl.java:635)
	at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$GitCommandMasterToSlaveCallable.call(RemoteGitImpl.java:170)
	at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler$GitCommandMasterToSlaveCallable.call(RemoteGitImpl.java:161)
	at hudson.remoting.UserRequest.perform(UserRequest.java:211)
	at hudson.remoting.UserRequest.perform(UserRequest.java:54)
	at hudson.remoting.Request$2.run(Request.java:377)
	at hudson.remoting.InterceptingExecutorService.lambda$wrap$0(InterceptingExecutorService.java:78)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:829)
	Suppressed: hudson.remoting.Channel$CallSiteStackTrace: Remote call to macos-node-i-0efc27fdcab43d410
		at hudson.remoting.Channel.attachCallSiteStackTrace(Channel.java:1787)
		at hudson.remoting.UserRequest$ExceptionResponse.retrieve(UserRequest.java:356)
		at hudson.remoting.Channel.call(Channel.java:1003)
		at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler.execute(RemoteGitImpl.java:153)
		at jdk.internal.reflect.GeneratedMethodAccessor430.invoke(Unknown Source)
		at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
		at java.base/java.lang.reflect.Method.invoke(Method.java:566)
		at org.jenkinsci.plugins.gitclient.RemoteGitImpl$CommandInvocationHandler.invoke(RemoteGitImpl.java:138)
		at com.sun.proxy.$Proxy87.execute(Unknown Source)
		at hudson.plugins.git.GitSCM.fetchFrom(GitSCM.java:997)
		at hudson.plugins.git.GitSCM.retrieveChanges(GitSCM.java:1241)
		at hudson.plugins.git.GitSCM.checkout(GitSCM.java:1305)
		at org.jenkinsci.plugins.workflow.steps.scm.SCMStep.checkout(SCMStep.java:136)
		at org.jenkinsci.plugins.workflow.steps.scm.SCMStep$StepExecutionImpl.run(SCMStep.java:101)
		at org.jenkinsci.plugins.workflow.steps.scm.SCMStep$StepExecutionImpl.run(SCMStep.java:88)
		at org.jenkinsci.plugins.workflow.steps.SynchronousNonBlockingStepExecution.lambda$start$0(SynchronousNonBlockingStepExecution.java:47)
		at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
		at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
		at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
		at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
		at java.base/java.lang.Thread.run(Thread.java:829)
ERROR: Error fetching remote repo 'origin'
ERROR: Maximum checkout retry attempts reached, aborting

[etcwilde]

@swift-ci please test macOS

[etcwilde]

The cherry-pick doesn't pass all of the flags, only the first two. I have llvm#133952 posted to LLVM upstream that will pass all of the flags to the try-compile. I'll cherry-pick that one onto this PR once I get it landed.

[etcwilde]

@swift-ci please test