versioner: follow __asm__ labels.

Attribute the versioning information on `void foo() __asm("bar")` to
bar, not foo.

The various long double functions in <math.h> run into this.

Bug: https://github.com/android-ndk/ndk/issues/440
Test: python run_tests.py
Test: m
Change-Id: Idd3681ddbd006b4705608449935c9cfacfa3556e