[AArch64] Fix the upper limit for folded address offsets for COFF

In COFF, the immediates in IMAGE_REL_ARM64_PAGEBASE_REL21 relocations
are limited to 21 bit signed, i.e. the offset has to be less than
(1 << 20). The previous limit did intend to cover for this case, but
had missed that the 21 bit field was signed.

This fixes issue https://github.com/llvm/llvm-project/issues/54753.

Differential Revision: https://reviews.llvm.org/D123160