[COFF, ARM64] Fix typo in COFF ARM64 Relocation Type

Summary:
Fixed IMAGE_REL_ARM64_PAGEBASE_REL2 ==> IMAGE_REL_ARM64_PAGEBASE_REL21
Refer: http://www.microsoft.com/whdc/system/platform/firmware/PECOFF.mspx

Reviewers: zturner, rnk, ruiu

Reviewed By: ruiu

Subscribers: aemerson, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D34659

llvm-svn: 306366