[ARM] create new pseudo t2LDRLIT_ga_pcrel for stack guards

We can't use the existing pseudo ARM::tLDRLIT_ga_pcrel for loading the
stack guard for PIC code that references the GOT, since arm-pseudo may
expand this to the narrow tLDRpci rather than the wider t2LDRpci.

Create a new pseudo, t2LDRLIT_ga_pcrel, and expand it to t2LDRpci.

Fixes: https://bugs.chromium.org/p/chromium/issues/detail?id=1270361

Reviewed By: ardb

Differential Revision: https://reviews.llvm.org/D114762