build: Use $(CCAS) for compiling .S files

build: Use $(CCAS) for compiling .S files

We fail to pass to $(AS) all of the different flags that may be required
for a given set of CFLAGS.  Rather than figuring out the host-specific
mapping, it's better to allow the compiler driver to do that.

However, simply using $(CC) runs afoul of clang trying to build the
option roms.  C.f. 3dd46c78525a30e98c68, wherein we changed from
using $(CC) to using $(AS) in the first place.

Work around this by passing -fno-integrated-as to clang, so that we use
the external assembler, and the clang driver still passes along all of
the options that the assembler might require.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <1466703558-7723-1-git-send-email-rth@twiddle.net>