[AArch64] Stop creating unnecessary label MCSymbols for each Windows unwind opcode. NFC.

These labels aren't needed in the ARM version of WinEH tables, as each
unwind opcode maps to a specific instruction (each opcode is assumed
to represent one instruction), and the written tables don't contain
offsets like on x86_64.

Differential Revision: https://reviews.llvm.org/D125369