AArch64AsmParser: Restore Lsym@page-offset support
https://github.com/llvm/llvm-project/pull/134202 removed support for
`sym@page-offset` in instruction operands. This change is generally
reasonable since subtracting an offset from a symbol typically doesn’t
make sense for Mach-O due to its .subsections_via_symbols mechanism, which treats
them as separate atoms.
However, BoringSSL relies on a temporary symbol with a negative offset,
which can be meaningful when the symbol and the referenced location are
within the same atom.
```
../../third_party/boringssl/src/gen/bcm/p256-armv8-asm-apple.S:1160:25: error: unexpected token in argument list
adrp x23,Lone_mont@PAGE-64
```
It's worth noting that expressions involving @ can be complex and
brittle in MCParser, and much of the Mach-O @ offsets remains
under-tested.
* Allow default argument for parsePrimaryExpr. The argument, used by the niche llvm-ml,
should not require other targets to adapt.
