libressl: limit --noexecstack to llvm

To avoid:
```
/usr/bin/x86_64-w64-mingw32-as: unrecognized option '--noexecstack'
```
Ref: https://github.com/curl/curl-for-win/actions/runs/6476061521/job/17584156242#step:3:4365

libressl-cmake.sh

@@ -16,7 +16,11 @@ _VER="$1"
 
   rm -r -f "${_PKGDIR:?}" "${_BLDDIR:?}"
 
-  CFLAGS="-ffile-prefix-map=$(pwd)= -Wa,--noexecstack"
+  CFLAGS="-ffile-prefix-map=$(pwd)="
+
+  if [ "${_CC}" = 'llvm' ]; then
+    CFLAGS="${CFLAGS} -Wa,--noexecstack"
+  fi
 
   [ "${_CPU}" = 'x86' ] && cpu='x86'
   [ "${_CPU}" = 'x64' ] && cpu='x86_64'

libressl.sh

@@ -18,14 +18,16 @@ _VER="$1"
 
   options="${_CONFIGURE_GLOBAL}"
   export CC="${_CC_GLOBAL}"
-  export CFLAGS="${_CFLAGS_GLOBAL} -O3 -Wa,--noexecstack"
+  export CFLAGS="${_CFLAGS_GLOBAL} -O3"
   export CPPFLAGS="${_CPPFLAGS_GLOBAL}"
   export LDFLAGS="${_LDFLAGS_GLOBAL} ${_LDFLAGS_GLOBAL_AUTOTOOLS}"
   export LIBS="${_LIBS_GLOBAL}"
 
   [ "${LIBRESSL_VER_}" = '3.8.1' ] && options="${options} --disable-asm"
 
-  if [ "${_CC}" = 'gcc' ]; then
+  if [ "${_CC}" = 'llvm' ]; then
+    CFLAGS="${CFLAGS} -Wa,--noexecstack"
+  else
     CFLAGS="${CFLAGS} -Wno-attributes"
   fi