gdb: fix shellcheck warnings SC2166 (&& and !! instead of -a and -o) in gdbarch.sh

gdb: fix shellcheck warnings SC2166 (&& and !! instead of -a and -o) in gdbarch.sh

Fix all warnings of this type:

    In gdbarch.sh line 1238:
        if [ "x${invalid_p}" = "x0" -a -n "${postdefault}" ]
                                    ^-- SC2166: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.

See the rationale here [1].

[1] https://github.com/koalaman/shellcheck/wiki/SC2166

gdb/ChangeLog:

	* gdbarch.sh: Use shell operators && and || instead of
	-a and -o.