Add a basic NDK compatibility library.

We know we can safely statically link `libm`, since it doesn't have
any dependencies on the OS or the layout of a data type that has
changed between releases (like `pthread_t`).

We can safely statically link `libc_syscalls` because the user can
check for and handle `ENOSYS`.

Update `ndk_missing_symbols.py` to account for symbols that are in the
compatibility library.

Improve `symbols.py` to be able to pull symbols from a static library.

Change-Id: Ifb0ede1e8b4a8f0f33865d9fed72fb8b4d443fbc