Move crt sources out of API specific directories.

We use one set of sources for all API levels, and the code to find
these made checkbuild.py messier than it needed to be. Move them into
a non-API level specific location.

Test: ndk/checkbuild.py && ndk/run_tests.py
Bug: None
Change-Id: I8e571037b761ebc64b0bf234abd90511778f3224