Stabilize TestGdbRemoteLibrariesSvr4Support

on some systems this test fails because the two methods it uses to
cross-reference the data don't match in the case of the vdso module. The
"read from /proc/%pid/maps" method returns "[vdso]", while the method
which reads it from the linker rendezvous structures returns
"linux-vdso.so.1". Neither of the two names match any actual file.

This restricts the test to only consider the libraries that we ourselves
have added to the test, minimizing the impact of system dependencies
that we cannot control.

llvm-svn: 363772