amd: flatten amd/common makefile structure

This pulls amd/common build rules into upper level makefile,
along with amd/addlib which is already there.

v2: [Emil Velikov]
 - Move NEED_RADEON_LLVM conditional, drop amd/common from SUBDIRS
 - Drop AM_ from common_libamd_common_la*

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
5 files changed