- eventfd.cpp and eventfd.s will output to the same file when building libc.a
out/target/product/*/obj/STATIC_LIBRARIES/libc_intermediates/WHOLE/libc_common_objs/eventfd.o
- And then `eventfd` will undefined when statically linked to libc.
Also add a unit test.
(cherry-pick of 8baa929d5d3bcf63381cf78ba76168c80c303f5e.)
Change-Id: Icd0eb0f4ce0511fb9ec00a504d491afd47d744d3