[CMake][libunwind] Fix the -target and -gcc-toolchain flag handling

CMake has the problem with the single dash variant because of the
space, so use the double dash with equal sign version. The compile
flag handling had a typo which caused these flag not to be properly
include. We also don't have to pass the target triple when checking
for compiler-rt since that flag is already included in compile flags
now.

Differential Revision: https://reviews.llvm.org/D32071

llvm-svn: 300419