[AArch64] Allow .variant_pcs before the symbol is registered

glibc sysdeps/aarch64/tst-vpcs-mod.S has something like:
```
.variant_pcs    vpcs_call
.global vpcs_call
```

This is supported by GNU as but leads to an error in MC. Use getOrCreateSymbol
to support a not-yet-registered symbol: call `registerSymbol` to ensure the
symbol exists even if there is no binding directive/label, to match GNU as.

While here, improve tests to check (1) a local symbol can get
STO_AARCH64_VARIANT_PCS (2) undefined .variant_pcs (3) an alias does not
inherit STO_AARCH64_VARIANT_PCS.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D122507