[ARM] Constant pools need 4-byte alignment if we only have tADR

When the only ADR instruction we have is the 16-bit thumb one then all
constant pool entries need to be 4-byte aligned, as tADR has an offset
that's a multiple of 4.

It looks like previously there happened to be no situations in which
we encountered a constant pool entry with alignment less than 4, so
failing to do this didn't cause any problems, but the expansion of
cttz to a table added by D128911 does use a constant pool with
alignment 1, so we now need to handle it correctly.

Differential Revision: https://reviews.llvm.org/D133199