The minimum supported Windows version is now Windows 7. Windows XP
and Windows Vista are no longer supported; both are already broken, and
require extra steps to use.
This commit removes the delayed-binding support for Windows API
functions that are present on all supported Windows targets. This has
several benefits: Removes needless complexity. Removes a load and
dynamic call on hot paths in mutex acquire / release. This may have
performance benefits.
* "Drop official support for Windows XP"
https://github.com/rust-lang/compiler-team/issues/378
* "Firefox has ended support for Windows XP and Vista"
https://support.mozilla.org/en-US/kb/end-support-windows-xp-and-vista