Issue #11729: Backport commit bff052d9 from libffi upstream in order to fix

a ctypes build failure with clang.

Modules/_ctypes/libffi/configure

@@ -14322,10 +14322,10 @@ if ${libffi_cv_as_x86_pcrel+:} false; then :
   $as_echo_n "(cached) " >&6
 else
 
-	libffi_cv_as_x86_pcrel=yes
+	libffi_cv_as_x86_pcrel=no
 	echo '.text; foo: nop; .data; .long foo-.; .text' > conftest.s
-	if $CC $CFLAGS -c conftest.s 2>&1 | $EGREP -i 'illegal|warning' > /dev/null; then
-	    libffi_cv_as_x86_pcrel=no
+	if $CC $CFLAGS -c conftest.s > /dev/null 2>&1; then
+	    libffi_cv_as_x86_pcrel=yes
 	fi
 
 fi

Modules/_ctypes/libffi/configure.ac

@@ -303,10 +303,10 @@ fi
 if test x$TARGET = xX86 || test x$TARGET = xX86_WIN32 || test x$TARGET = xX86_64; then
     AC_CACHE_CHECK([assembler supports pc related relocs],
 	libffi_cv_as_x86_pcrel, [
-	libffi_cv_as_x86_pcrel=yes
+	libffi_cv_as_x86_pcrel=no
 	echo '.text; foo: nop; .data; .long foo-.; .text' > conftest.s
-	if $CC $CFLAGS -c conftest.s 2>&1 | $EGREP -i 'illegal|warning' > /dev/null; then
-	    libffi_cv_as_x86_pcrel=no
+	if $CC $CFLAGS -c conftest.s > /dev/null 2>&1; then
+	    libffi_cv_as_x86_pcrel=yes
 	fi
 	])
     if test "x$libffi_cv_as_x86_pcrel" = xyes; then