Remove morestack support

This commit removes all morestack support from the compiler which entails:

* Segmented stacks are no longer emitted in codegen.
* We no longer build or distribute libmorestack.a
* The `stack_exhausted` lang item is no longer required

The only current use of the segmented stack support in LLVM is to detect stack
overflow. This is no longer really required, however, because we already have
guard pages for all threads and registered signal handlers watching for a
segfault on those pages (to print out a stack overflow message). Additionally,
major platforms (aka Windows) already don't use morestack.

This means that Rust is by default less likely to catch stack overflows because
if a function takes up more than one page of stack space it won't hit the guard
page. This is what the purpose of morestack was (to catch this case), but it's
better served with stack probes which have more cross platform support and no
runtime support necessary. Until LLVM supports this for all platform it looks
like morestack isn't really buying us much.

cc #16012 (still need stack probes)
Closes #26458 (a drive-by fix to help diagnostics on stack overflow)

Author: alexcrichton

mk/clean.mk

@@ -101,7 +101,6 @@ define CLEAN_TARGET_STAGE_N
 clean$(1)_T_$(2)_H_$(3): \
 	    $$(foreach crate,$$(CRATES),clean$(1)_T_$(2)_H_$(3)-lib-$$(crate)) \
 	    $$(foreach tool,$$(TOOLS) $$(DEBUGGER_BIN_SCRIPTS_ALL),clean$(1)_T_$(2)_H_$(3)-tool-$$(tool))
-	$$(Q)rm -f $$(TLIB$(1)_T_$(2)_H_$(3))/libmorestack.a
 	$$(Q)rm -f $$(TLIB$(1)_T_$(2)_H_$(3))/libcompiler-rt.a
 	$(Q)rm -f $$(TLIB$(1)_T_$(2)_H_$(3))/librun_pass_stage* # For unix
 	$(Q)rm -f $$(TLIB$(1)_T_$(2)_H_$(3))/run_pass_stage* # For windows

mk/crates.mk

@@ -65,7 +65,7 @@ DEPS_libc := core
 DEPS_rustc_unicode := core
 DEPS_alloc := core libc native:jemalloc
 DEPS_std := core libc rand alloc collections rustc_unicode \
-	native:rust_builtin native:backtrace native:rustrt_native \
+	native:rust_builtin native:backtrace \
 	rustc_bitflags
 DEPS_graphviz := std
 DEPS_syntax := std term serialize log fmt_macros arena libc