Driver: Refactor and support per target dirs in baremetal

Refactor baremetal driver code to reduce the bespoke
additions and base class overrides.
This lets us use the per target runtimes like other clang
targets. E.g. clang -target armv7m-cros-none-eabi will now
be able to use the runtimes installed at
<resource_dir>/lib/armv7m-cros-none-eabi instead of the hardcoded
path <resource_dir>/lib/baremetal.
The older code paths should still continue to work as before if
<resource_dir>/lib/<tuple> does not exist.

Reviewed By: MaskRay, barannikov88

Differential Revision: https://reviews.llvm.org/D131225