Use unadjusted ABI and restore intra-doc links with crate::arch::x86

Switch the llvm.x86.clflushopt extern block from "C" to "unadjusted" to
match the convention for LLVM intrinsics (movrs, kl, adx, rdrand) — the
unadjusted ABI passes the signature 1:1 with no C-ABI adjustments.

Restore the _mm_clflush/_mm_sfence/_mm_mfence intra-doc link definitions
using crate::arch::x86 (not x86_64), which resolves on all targets and
avoids the i686 rustdoc broken-intra-doc-links lint. The links are needed
because this module has no glob import to bring them into scope.

Author: jlpetz

crates/core_arch/src/x86/clflushopt.rs

@@ -4,7 +4,7 @@
 use stdarch_test::assert_instr;
 
 #[allow(improper_ctypes)]
-unsafe extern "C" {
+unsafe extern "unadjusted" {
     #[link_name = "llvm.x86.clflushopt"]
     fn clflushopt(p: *const u8);
 }
@@ -26,6 +26,10 @@ unsafe extern "C" {
 /// Unlike the prefetch intrinsics, `CLFLUSHOPT` is subject to all the
 /// permission checking and faults associated with a byte load, so `p` must
 /// point to a byte that is valid for reads.
+///
+/// [`_mm_clflush`]: crate::arch::x86::_mm_clflush
+/// [`_mm_sfence`]: crate::arch::x86::_mm_sfence
+/// [`_mm_mfence`]: crate::arch::x86::_mm_mfence
 #[inline]
 #[target_feature(enable = "clflushopt")]
 #[cfg_attr(test, assert_instr(clflushopt))]