x86/uaccess: Remove .fixup usage

For the !CC_AS_ASM_GOTO_OUTPUT (aka. the legacy codepath), remove the
.fixup usage by employing both EX_TYPE_EFAULT_REG and EX_FLAG_CLEAR.
Like was already done for X86_32's version of __get_user_asm_u64() use
the "a" register for output, specifically so we can use CLEAR_AX.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20211110101325.485154848@infradead.org