Fix comment in AArch64 ELF backend


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222814 91177308-0d34-0410-b5e6-96231b3b80d8
Rename R_AARCH64_TLSDESC_ADR_PAGE to R_AARCH64_TLSDESC_ADR_PAGE21


git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@222823 91177308-0d34-0410-b5e6-96231b3b80d8