[Driver] Allow target override containing . in executable name

The gcc compatible driver has support for overriding the default
target based on the driver's executable name, for instance
x86_64-pc-linux-gnu-clang will set the default target to
x86_64-pc-linux-gnu.

Previously, this failed when the target contained a minor version, for
example x86_64-pc-freebsd13.1, so instead of finding the file's
stem, use the whole file name, but strip off any '.exe' from the tail.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D135284