arm64: start using 'asm goto' for get_user() when available

This generates noticeably better code with compilers that support it,
since we don't need to test the error register etc, the exception just
jumps to the error handling directly.

Note that this also marks SW_TTBR0_PAN incompatible with KCSAN support,
since KCSAN wants to save and restore the user access state.

KCSAN and SW_TTBR0_PAN were probably always incompatible, but it became
obvious only when implementing the unsafe user access functions.  At
that point the default empty user_access_save/restore() functions
weren't provided by the default fallback functions.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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