This reverts commit 89e8ae6a.

Previously mirrors were stored in the rust-lang-ci2 bucket, which is
meant to store temporary data (the CI artifacts). This switches the code
to fetch from the new mirrors bucket.

The old files won't be removed, but they won't be backed up either.

libc currently denies all warnings by default. This commit denies
warnings only when libc is built in CI.

2. change armv7-wrs-vxworks to armv7-wrs-vxworks-eabihf
3. code cleanup

Define the C interface to vxWorks for x86, x86_64, ARM, ARM 64-bit, PowerPC, PowerPC SPE and PowerPC 64-bit.

 Move to LINUX_NO_CORE_TARGETS per gznlbg's suggestion.

Move to RUST_NIGHTLY_LINUX_TARGETS per gznlbg's suggestion.