refactor(build): pass TF_CFLAGS to the assembler

The assembler is already invoked through the gcc/llvm wrapper so will
understand the TF_CFLAGS out of the box. TF_CFLAGS are also quite
comprehensive. So don't duplicate the definitions and use it directly.
This also allows to absorb TF_CFLAGS_$(ARCH) and CPPFLAGS and reduce
the number of permutations of flags we pass.

Change-Id: I801cea0421dab5a07bf720be9693dce3ef220dcf
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
2 files changed