[Driver] Recognize powerpc-unknown-eabi as a bare-metal toolchain

[Driver] Recognize powerpc-unknown-eabi as a bare-metal toolchain

This seems to match https://gcc.gnu.org/install/specific.html#powerpc-x-eabi

It seems that anything with OS `none` (although that doesn’t seem to be distinguished from `unknown`) or with environment `eabi` should be treated as bare-metal.
Since this seems to have been handled on a case-by-case basis in the past ([arm](https://reviews.llvm.org/D33259), [riscv](https://reviews.llvm.org/D91442), [aarch64](https://reviews.llvm.org/D111134)), what I am proposing here is to add another case to the list to also handle `powerpc[64][le]-unknown-unknown-eabi` using the `BareMetal` toolchain, following the example of the existing cases. (We don’t care about powerpc64 and powerpc[64]le, but it seemed appropriate to lump them in.)

At Indel, we have been building bare-metal embedded applications that run on custom PowerPC and ARM systems with Clang and LLD for a couple of years now, using target triples `powerpc-indel-eabi`, `powerpc-indel-eabi750`, `arm-indel-eabi`, `aarch64-indel-eabi` (which I just learned from D153430 is wrong and should be `aarch64-indel-elf` instead, but that’s a different matter). This has worked fine for ARM, but for PowerPC we have been unable to call the linker (LLD) through the Clang driver, because it would insist on calling GCC as the linker, even when told `-fuse-ld=lld`. That does not work for us, there is no GCC around. Instead we had to call `ld.lld` directly, introducing some special cases in our build system to translate between linker-via-driver and linker-called-directly command line arguments. I have now dug into why that is, and found that the difference between ARM and PowerPC is that `arm-indel-eabi` hits a special case that causes the Clang driver to instantiate a `BareMetal` toolchain that is able to call LLD and works the way we need, whereas `powerpc-indel-eabi` lands in the default case of a `Generic_ELF` (subclass of `Generic_GCC`) toolchain which expects GCC.

Reviewed By: MaskRay, michaelplatings, #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D154357