AArch64: estimate inline asm length during branch relaxation

To make sure branches are in range, we need to do a better job of estimating
the length of an inline assembly block than "it's probably 1 instruction, who'd
write asm with more than that?".

Fortunately there's already a (highly suspect, see how many ways you can think
of to break it!) callback for this purpose, which is used by the other targets.

rdar://problem/17277590