[MC][COFF][AArch64] Avoid incorrect IMAGE_REL_ARM64_BRANCH26 relocations.

For a b/bl instruction that branches a temporary symbol (private
assembly label), an IMAGE_REL_ARM64_BRANCH26 relocation to another
symbol + a non-zero offset could be emitted but the linkers don't
support this type of relocation and cause incorrect relocations and
crashes. Avoid emitting this type of relocations.

Differential Revision: https://reviews.llvm.org/D155732