refactor(build): merge march32/64 directives

Both march32-directive and march64-directive eventually generate the
same march option that will passed to compiler.

Merge this two separate directives to a common one as march-directive.

Change-Id: I220d2b782eb3b54e13ffd5b6a581d0e6da68756a
Signed-off-by: Govindraj Raja <govindraj.raja@arm.com>
4 files changed