Target sanity-check in bootstrap incorrect when doing a local-rebuild

[bjorn3]

When doing a local-rebuild, the bootstrap compiler is supposed to be functionally identical to the compiler we are building and thus supports all newly introduced targets. As such all targets in the STAGE0_MISSING_TARGETS list would actually be supported. With current nightly doing a local-rebuild results in the following error due to this issue:

Following targets supported from the stage0 compiler, please remove them from `STAGE0_MISSING_TARGETS` list.
  armv7-rtems-eabihf
Build completed unsuccessfully in 0:00:28

We should probably set BOOTSTRAP_SKIP_TARGET_SANITY=1 unconditionally for all local-rebuilds, or alternatively pretend STAGE0_MISSING_TARGETS is empty.

[bjorn3]

Worked around this in rust-lang/rustc_codegen_cranelift@ade0e38.