configure: Only use gas-preprocessor with armasm for compiler=CL

This picks the right assembler automatically for arm and aarch64
llvm-mingw targets.

This doesn't get the right assembler for clang setups when clang
acts like MSVC and uses MSVC headers though (where it perhaps
should use armasm as before), but that's probably an even more
obscure setup.

configure

@@ -798,8 +798,10 @@ case $host_cpu in
                 CFLAGS="$CFLAGS -arch armv7"
                 LDFLAGS="$LDFLAGS -arch armv7"
             fi
-        elif [ "$SYS" = WINDOWS ] ; then
+        elif [ "$SYS" = WINDOWS ] && [ "$compiler" = CL ] ; then
             AS="${AS-${SRCPATH}/tools/gas-preprocessor.pl -arch arm -as-type armasm -force-thumb -- armasm -nologo -ignore 4509}"
+        elif [ "$SYS" = WINDOWS ] ; then
+            AS="${AS-${SRCPATH}/tools/gas-preprocessor.pl -arch arm -as-type clang -force-thumb -- ${CC} -mimplicit-it=always}"
         else
             AS="${AS-${CC}}"
         fi
@@ -810,7 +812,7 @@ case $host_cpu in
         if [ "$SYS" = MACOSX ] ; then
             AS="${AS-${CC}}"
             ASFLAGS="$ASFLAGS -DPREFIX -DPIC"
-        elif [ "$SYS" = WINDOWS ] ; then
+        elif [ "$SYS" = WINDOWS ] && [ "$compiler" = CL ] ; then
             AS="${AS-${SRCPATH}/tools/gas-preprocessor.pl -arch aarch64 -as-type armasm -- armasm64 -nologo}"
         else
             AS="${AS-${CC}}"