Reimplement _xgetbv with LLVM intrinsics (#958)

`_xgetbv` was reimplemented to use inline assembly in #333 since LLVM
3.9 didn't export the intrinsic we needed to use.  LLVM 4.0 has since
rectified that issue, and since rust's minimum supported version of LLVM
is 8.0, this change can be reverted.

Author: sadlerap

crates/core_arch/src/x86/xsave.rs

@@ -12,6 +12,8 @@ extern "C" {
     fn xrstor(p: *const u8, hi: u32, lo: u32);
     #[link_name = "llvm.x86.xsetbv"]
     fn xsetbv(v: u32, hi: u32, lo: u32);
+    #[link_name = "llvm.x86.xgetbv"]
+    fn xgetbv(v: u32) -> i64;
     #[link_name = "llvm.x86.xsaveopt"]
     fn xsaveopt(p: *mut u8, hi: u32, lo: u32);
     #[link_name = "llvm.x86.xsavec"]
@@ -85,10 +87,7 @@ pub unsafe fn _xsetbv(a: u32, val: u64) {
 #[cfg_attr(test, assert_instr(xgetbv))]
 #[stable(feature = "simd_x86", since = "1.27.0")]
 pub unsafe fn _xgetbv(xcr_no: u32) -> u64 {
-    let eax: u32;
-    let edx: u32;
-    llvm_asm!("xgetbv" : "={eax}"(eax), "={edx}"(edx) : "{ecx}"(xcr_no));
-    ((edx as u64) << 32) | (eax as u64)
+    xgetbv(xcr_no) as u64
 }
 
 /// Performs a full or partial save of the enabled processor states to memory at