Avoid using TARGET_ARCH in llvm.build.mk

Apparently the TARGET_ARCH macro is not supposed to be used outside of
the top-level Makefiles. Directly use MACHINE_ARCH instead.

Noticed by: imp, jrtc27
MFC after: 1 week

(cherry picked from commit 8534e6be8110a8126268a38dc0557a2d15615ce9)