Fix an issue in some Thumb fixups, where the effective PC address needs to be 4-byte...

Fix an issue in some Thumb fixups, where the effective PC address needs to be 4-byte aligned when calculating
the offset.  Add a new fixup flag to represent this, and use it for the one fixups that I have a testcase for needing
this.  It's quite likely that the other Thumb fixups will need this too, and to have their fixup encoding logic
adjusted accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121408 91177308-0d34-0410-b5e6-96231b3b80d8