Add a non-TLS slot implementation of locale.cpp.

libandroid_support shouldn't be using Bionic's TLS slot. Add a
thread_local based implementation. We can't use the thread_local
implementation in bionic because the linker needs this and pthread
thread-locals (which is how thread_local is implemented) doesn't work
that early.

Test: make checkbuild
Test: adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
Bug: https://github.com/android-ndk/ndk/issues/300
Change-Id: I7dcbf554ade2264d541d722fa3f86df04926e67a