Ideally the official build system should have an easy way to mix ASM .s files with .c source code. This would allow motivated developers to easily optimize at the ASM level. (.s files are generally cleaner and more efficient than inline ASM if developed by someone who understands the ABI).
I worked out a solution, but it requires modifying C_API/buildsupport/common.mk . See the following thread. Note that C_API/Examples/3D library/Makefile has a line for ASRC that does not appear to be being used.
I suppose any given project could use a custom Makefile based on the official common.mk.