Rollup merge of #64051 - alex:linux-kernel-module-target, r=joshtriplett

Add x86_64-linux-kernel target

This adds a target specification for Linux kernel modules on x86_64, as well as base code that can be shared with other architectures.

I wasn't totally sure about what the best name for this was.

There's one open question on whether we should use the LLVM generic x86_64-elf target, or the same one used for the Linux userspace.

r? @joshtriplett