Remove xbegin and _xend

Author: DKLoehr

clang/lib/Headers/intrin.h

@@ -162,8 +162,6 @@ void _Store_HLERelease(long volatile *, long);
 void _Store64_HLERelease(__int64 volatile *, __int64);
 void _StorePointer_HLERelease(void *volatile *, void *);
 void _WriteBarrier(void);
-unsigned __int32 xbegin(void);
-void _xend(void);
 
 /* These additional intrinsics are turned on in x64/amd64/x86_64 mode. */
 #if defined(__x86_64__) && !defined(__arm64ec__)