ASan: unbreak Windows build by limiting backtrace* tests to glibc

My newly added backtrace test (https://reviews.llvm.org/D150491)
broke the Windows buildbot (https://lab.llvm.org/buildbot/#/builders/127/builds/48103)
because they do not have execinfo.h.
I expect the same will happen with the backtrace_symbols test (https://reviews.llvm.org/D150498) as well.

This patch does a quick fix by restricting the test scope to glibc-2.27.
(A tricker fix would take into account SANITIZER_INTERCEPT_BACKTRACE,
which is defined as (SI_FREEBSD || SI_NETBSD || SI_GLIBC || SI_SOLARIS))