x86/uaccess: utilize CONFIG_CC_HAS_ASM_GOTO_OUTPUT

Clang-11 shipped support for outputs to asm goto statments along the
fallthrough path.  Double up some of the get_user() and related macros
to be able to take advantage of this extended GNU C extension. This
should help improve the generated code's performance for these accesses.

Cc: Bill Wendling <morbo@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>