librustc: back::link::sanitize support esacpe_utf8
librustc: apply changes of char::escape_unicode
auto merge of #7488 : yichoi/rust/sanitize_utf8, r=huonw

back::link::sanitize support escape_utf8
fix #7486
Turn on using LLVM threadsafely
auto merge of #7409 : alexcrichton/rust/threadsafe, r=cmr

@catamorphism, this re-enables threadsafe rustpkg tests, @brson this will fail unless the bots have LLVM rebuilt, so this is a good indicator of whether that happened or not.
Ignore all rusti tests. #7541
Ignore all rusti tests harder. #7541
Add x64 windows to platform.mk and mingw64 header fixes.