util_libc: Add a note about the x32 ABI.

Author: briansmith

src/util_libc.rs

@@ -109,8 +109,6 @@ pub fn getrandom_syscall(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
     // no real truncation here, even when running
     debug_assert!(buf.len() <= isize::MAX.unsigned_abs());
 
-    // TODO: Add support for x32 and similar ABIs?
-
     // pointers can be passed to libc::syscall.
     const _: () = assert!(
         core::mem::size_of::<libc::c_long>() == core::mem::size_of::<*mut core::ffi::c_void>()
@@ -124,6 +122,8 @@ pub fn getrandom_syscall(buf: &mut [MaybeUninit<u8>]) -> libc::ssize_t {
 
     const ZERO: libc::c_ulong = 0;
 
+    // Note that the `libc` crate adds `__X32_SYSCALL_BIT` for x32 targets.
+
     let res: libc::c_long = unsafe { libc::syscall(libc::SYS_getrandom, ptr, len, ZERO) };
 
     // c_long to ssize_t conversion is lossless.