configure: Also try -mstack-alignment for clang,

Fixes stack alignment on x86_32 FreeBSD.
Fixes ticket #5231.

configure

@@ -6202,6 +6202,7 @@ elif enabled llvm_gcc; then
     check_cflags -mllvm -stack-alignment=16
 elif enabled clang; then
     check_cflags -mllvm -stack-alignment=16
+    check_cflags -mstack-alignment=16
     check_cflags -Qunused-arguments
     check_cflags -Werror=implicit-function-declaration
     check_cflags -Werror=missing-prototypes