Copy CFLAGS to ASMFLAGs to enable CET in asm files

See here for the relevant equivalent of this change in Fedora:

8e1f53d786

Resolves: rhbz#2118567

llvm.spec

@@ -271,6 +271,9 @@ mv %{cmake_srcdir} cmake
 %global optflags %(echo %{optflags} | sed 's/-g /-g1 /')
 %endif
 
+# Copy CFLAGS into ASMFLAGS, so -fcf-protection is used when compiling assembly files.
+export ASMFLAGS=$CFLAGS
+
 # force off shared libs as cmake macros turns it on.
 %cmake	-G Ninja \
 	-DBUILD_SHARED_LIBS:BOOL=OFF \