Make android_load_sphal_library thread-safe

Also: remove an obsolete comment from linker_test.cpp. The test program is
at /data/nativetest[64]/libvndksupport-tests/libvndksupport-tests, which
may will be in the system or legacy configuration, and either the "sphal"
or "default" namespace will be visible.

Bug: none
Test: run libvndksupport-tests, device boots
Change-Id: I5e33e5bf1402a0368e046b03827c216789b02c96