libc: Add default __STRING implementation

This commit defines a default `__STRING()` definition.  There should be
a default definition because the unit test cases of versioner will
include `android/versioning.h` directly.  We cannot include
`sys/cdefs.h` in `android/versioning.h` either because that will lead to
circular inclusion.

Bug: 113263746
Test: ./bionic/tools/versioner/run_tests.py
Change-Id: Ie99c0eef4414fd687347baf3a2d295468877464f