build: filter out command line assembler arguments

build: filter out command line assembler arguments

If the assembler is not used. This happens when using cc -E or cc -S
for example. GCC will just ignore the -Wa,... when the assembler is
not called, but clang will complain loudly and fail.

Also enable passing -Wa,-I$(BASEDIR)/include to clang now that it's
safe to do so.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>