[lld-macho][nfc] define command UNWIND_MODE_MASK for convenience and rewrite...

[lld-macho][nfc] define command UNWIND_MODE_MASK for convenience and rewrite mode-mask checking logic for clarity

The previous form is currently "harmless" and happened to work but may not in the future:

Consider the struct: (for x86-64, but same issue can be said for the ARM/64 families):

```
UNWIND_X86_64_MODE_MASK                    = 0x0F000000,
UNWIND_X86_64_MODE_RBP_FRAME               = 0x01000000,
UNWIND_X86_64_MODE_STACK_IMMD              = 0x02000000,
UNWIND_X86_64_MODE_STACK_IND               = 0x03000000,
UNWIND_X86_64_MODE_DWARF                   = 0x04000000,
```

Previously, we were doing: `(encoding & MODE_DWARF) == MODE_DWARF`

As soon as a new `UNWIND_X86_64_MODE_FOO = 0x05000000` is defined, then the check above would always return true for encoding=MODE_FOO (because `(0b0101 & 0b0100) == 0b0100` )

Differential Revision: https://reviews.llvm.org/D135359