Merge branch 'arm64-uaccess' (early part)

Merge arm64 support for proper 'unsafe' user accessor functionality,
with 'asm goto' for handling exceptions.

The arm64 user access code used the slow fallback code for the user
access code, which generates horrendous code for things like
strncpy_from_user(), because it causes us to generate code for SW PAN
and for range checking for every individual word.

Teach arm64 about 'user_access_begin()' and the so-called 'unsafe' user
access functions that take an error label and use 'asm goto' to make all
the exception handling be entirely out of line.

[ These user access functions are called 'unsafe' not because the
  concept is unsafe, but because the low-level accessor functions
  absolutely have to be protected by the 'user_access_begin()' code,
  because that's what does the range checking.

  So the accessor functions have that scary name to make sure people
  don't think they are usable on their own, and cannot be mis-used the
  way our old "double underscore" versions of __get_user() and friends
  were ]

The "(early part)" of the branch is because the full branch also
improved on the "access_ok()" function, but the exact semantics of TBI
(top byte ignore) have to be discussed before doing that part.  So this
just does the low-level accessor update to use "asm goto".

* 'arm64-uaccess' (early part):
  arm64: start using 'asm goto' for put_user()
  arm64: start using 'asm goto' for get_user() when available

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