libgo: use inline assembly in favor of call to _xgetbv()

    
    Use inline assembly in the implementation of internal_cpu.xgetbv as
    opposed to a call to the intrinsic _xgetbv(), since non-gcc compilers
    (e.g. clang) may or may not have support for it.
    
    Reviewed-on: https://go-review.googlesource.com/c/140137

From-SVN: r264882

gcc/go/gofrontend/MERGE

-9f4cf23e716bcf65e071260afa032a64acd3fdde
+d0739c13ca3686df1f8d0fae7c6c5caaed058503
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/internal/cpu/cpu_gccgo.c

@@ -52,12 +52,18 @@ struct xgetbv_ret xgetbv(void)
 #pragma GCC target("xsave")
 
 struct xgetbv_ret xgetbv(void) {
-	long long r;
 	struct xgetbv_ret ret;
 
-	r = _xgetbv(0);
-	ret.eax = r & 0xffffffff;
-	ret.edx = r >> 32;
+        // At some point, use call to _xgetbv() instead:
+        //
+        //       long long r = _xgetbv(0);
+        //       ret.eax = r & 0xffffffff;
+        //       ret.edx = r >> 32;
+        //
+        unsigned int __eax, __edx, __xcr_no = 0;
+        __asm__ ("xgetbv" : "=a" (__eax), "=d" (__edx) : "c" (__xcr_no));
+        ret.eax = __eax;
+        ret.edx = __edx;
 	return ret;
 }