fix(build): partially fix qemu aarch32 build

While aarch32 isn't currently supported on qemu, platform.mk contains
hard-coded references to aarch64 in BL1_SOURCES which should be ${ARCH}.

This improves the situation, but since aarch32/qemu_max.S doesn't exist
and there are other missing files for aarch32, this is only a partial
fix.

Signed-off-by: Rebecca Cran <rebecca@quicinc.com>
Change-Id: I3fa01483e572abfd781ceaecff16ecf57cda8316