x86/word-at-a-time: Remove .fixup usage

Rewrite load_unaligned_zeropad() to not require .fixup text.

This is easiest done using asm-goto-output, where we can stick a C
label in the exception table entry. The fallback version isn't nearly
so nice but should work.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20211110101326.141775772@infradead.org