[PATCH] x86_64: Improve Intel MPX check to reject incomplete Clang support

[Fangrui Song]

LLVM integrated assembler has incomplete MPX support, e.g.
bnd is not supported while bndmov is supported:

    error: invalid instruction mnemonic 'bnd'

Improve the configure check so that HAVE_MPX_SUPPORT code can be
properly skipped.
---
 sysdeps/x86_64/configure    | 2 ++
 sysdeps/x86_64/configure.ac | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/sysdeps/x86_64/configure b/sysdeps/x86_64/configure
index 585279f83d..c3145e6030 100755
--- a/sysdeps/x86_64/configure
+++ b/sysdeps/x86_64/configure
@@ -30,6 +30,8 @@ if ${libc_cv_asm_mpx+:} false; then :
 else
   cat > conftest.s <<\EOF
         bndmov %bnd0,(%rsp)
+        bnd
+        jmp *%r11
 EOF
 if { ac_try='${CC-cc} -c $ASFLAGS conftest.s 1>&5'
   { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
diff --git a/sysdeps/x86_64/configure.ac b/sysdeps/x86_64/configure.ac
index 29e14033c0..3c935d66e1 100644
--- a/sysdeps/x86_64/configure.ac
+++ b/sysdeps/x86_64/configure.ac
@@ -14,6 +14,8 @@ dnl Check whether asm supports Intel MPX
 AC_CACHE_CHECK(for Intel MPX support, libc_cv_asm_mpx, [dnl
 cat > conftest.s <<\EOF
         bndmov %bnd0,(%rsp)
+        bnd
+        jmp *%r11
 EOF
 if AC_TRY_COMMAND(${CC-cc} -c $ASFLAGS conftest.s 1>&AS_MESSAGE_LOG_FD); then
   libc_cv_asm_mpx=yes
-- 
2.33.0.882.g93a45727a2-goog