[compiler-rt] Disable coverage trace pc guard tests on Thumb

These are failing on our silent bot:
https://lab.llvm.org/staging/#/builders/162/builds/358

$ <run cmd>
main
foo
bar
baz
SanitizerCoverage: ./sanitizer_coverage_trace_pc_guard-dso.cpp.tmp.2122517.sancov: 2 PCs written
SanitizerCoverage: ./sanitizer_coverage_trace_pc_guard-dso.cpp.tmp_2.so.2122517.sancov: 1 PCs written
SanitizerCoverage: ./sanitizer_coverage_trace_pc_guard-dso.cpp.tmp_1.so.2122517.sancov: 1 PCs written
$ <sancov cmd>
ERROR: Coverage points in binary and .sancov file do not match.

Also reproduces if you build for Thumb on v8 hardware.

Doesn't fail when built with Arm only code so I guess the Thumb mode bit
in the PCs might be the issue.