aarch64 gas: use bool return type for sub-option parsing

The signature of the functions used to decode sub-options mirrors that
of md_parse_option(). They currently return an integer, but the return
value is always set to 0 on failure and 1 on success, which exactly
matches boolean semantics.
These functions likely predate C99 as the minimum supported C standard
in binutils. Today, there is no good reason to keep this legacy interface
instead of using a proper boolean type.

This patch updates the sub-option parsing functions to return a boolean
and adjusts the corresponding code accordingly.