build: always use the C compiler to assemble

The ROM library is the only component in the repository that compiles
assembly files using the assembler directly. This change migrates it to
the C compiler instead, like the rest of the project.

Change-Id: I6c50660eeb9be2ca8dcb0e626c37c197466b0fa1
Signed-off-by: Chris Kay <chris.kay@arm.com>
2 files changed