[compiler-rt] Mark symbolize_stack_fp test unsupported on Arm Thumb

The new test `symbolize_stack_fp.cpp` added in
https://reviews.llvm.org/D102046 assumes that
we can fall back to the fast unwinder.

This is not the case for Thumb and the test is currently
failing on our v7 thumb bot:
https://lab.llvm.org/buildbot/#/builders/26/builds/2096

Skip the test if we're building for a Thumb target.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D103512