[VE] Target-specific bit size for sjljehprepare

Summary:
This patch extends the TargetMachine to let targets specify the integer size
used by the sjljehprepare pass. This is 64bit for the VE target and otherwise
defaults to 32bit for all targets, which was hard-wired before.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D71337