[AArch64] Return the correct size for TLSDESC_CALLSEQ

The branch relaxation pass is computing the wrong offsets because it assumes
TLSDESC_CALLSEQ eats up 4 bytes, when in fact it is lowered to an instruction
sequence taking up 16 bytes. This can become a problem in huge files with lots
of TLS accesses, as it may slowly move branch targets out of the range computed
by the branch relaxation pass.

Fixes PR24234 https://llvm.org/bugs/show_bug.cgi?id=24234

Differential Revision: https://reviews.llvm.org/D22870

llvm-svn: 277331