CMake: make the regexes used for setting HOST_LINK_VERSION more forgiving (PR21268)

If the output of 'ld -v' didn't match the regexes, CMake would previously error
with a message like:

  CMake Error at tools/clang/CMakeLists.txt:269 (string):
    string sub-command REGEX, mode REPLACE regex "[^0-9]*([0-9.]*).*" matched
    an empty string.

llvm-svn: 223106