[Compiler-RT] Remove dependency on libgcc_s/eh

Currently, for --rtlib=compiler-rt on GNU systems, we're assuming
that one has libgcc_s and libgcc_eh as low-level libraries, which
when used in conjunction with -lunwind or -lc++abi, breaks that
assumption.

My original fix was wrong, and this patch reverts it to prepare for
a new flag to choose the unwinder/C++ libraries. For the time being,
people can use "-lgcc_s -lgcc_eh" or "-lunwind -lc++abi" or any
combination they need explicitly.

llvm-svn: 243025