[Clang][Driver] Warn on invalid Arm or AArch64 baremetal target triple

A common user mistake is specifying a target of aarch64-none-eabi or
arm-none-elf whereas the correct names are aarch64-none-elf &
arm-none-eabi. Currently if a target of aarch64-none-eabi is specified
then the Generic_ELF toolchain is used, unlike aarch64-none-elf which
will use the BareMetal toolchain. This is unlikely to be intended by the
user so issue a warning that the target is invalid.

The target parser is liberal in what input it accepts so invalid triples
may yield behaviour that's sufficiently close to what the user intended.
Therefore invalid triples were used in many tests. This change updates
those tests to use valid triples.
One test (gnu-mcount.c) relies on the Generic_ELF toolchain behaviour so
change it to explicitly specify aarch64-unknown-none-gnu as the target.

Reviewed By: peter.smith, DavidSpickett

Differential Revision: https://reviews.llvm.org/D153430