Revert "arm64: Avoid redundant type conversions in xchg() and cmpxchg()"

This reverts commit 73152d61.

The LL/SC atomics are exported to modules and so we cannot break the
ABI by changing their names. Since the original commit just removes some
redundant masking operations, there is no functional impact in reverting
it to maintain the ABI.

Cc: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ibebe6a63ead13ca566e0e0ff7e58eb325119f620
Signed-off-by: Will Deacon <willdeacon@google.com>