mk: Split target triples into bulid triple + host triples + target triples

For cross compiling to targets that don't want to build a compiler
mk: Refactor platform.mk a bit
submodule: libuv for android
mk: rewrite make files
mk: mingw32 flags added
mk: mingw32 fix
mk: Cross-compile fixes
Allow constant c-like enum to integral/float cast
mk: cleanup - lib and executable suffix handling
mk: rt.mk fix for libuv cross compile
mk: Restore toolchain config for i386 mac/linux, and freebsd

Conflicts:
	mk/platform.mk
mk: platform.mk more flags added
mk: platform.mk typo UNIXY
mk: install/uninstall for multiple target
mk: --android-cross-path to rustc
Finish de-implicit-selifizng libcore
Fix bug in coherence that causes all cross-crate impls to be regarded as
inherent impls, not just those of the `impl Type` variety.
Make object types not implement associated trait.  Fixes #5087.
Remove trailing whitespace.
auto merge of #5251 : bstrie/rust/deimpselfcore2, r=pcwalton