Put bsd_signal back.

With this change (and all the others below it in the stack), the only
difference between the arm android-9 libc.so in r13-beta1 and the one
we are now generating is the addition of LIBC_N and LIBC_O versions.

Test: make ndk, readelf stub libc.so to check symbol exists
Bug: https://github.com/android-ndk/ndk/issues/160
Change-Id: Iedab32592b2d979f3cc922ffd4ed406427de3dda