Always search sysroot for GCC installs

Previously, if clang was configured with -DGCC_INSTALL_PREFIX, then it
would not search a provided sysroot for a gcc install. This caused a
number of regression tests to fail.  If a sysroot is given, skip
searching GCC_INSTALL_PREFIX as it is likely not valid for the
provided sysroot.

llvm-svn: 344901