gdb: fix shellcheck warnings SC2006 (use $() instead of ``) in gdbarch.sh

gdb: fix shellcheck warnings SC2006 (use $() instead of ``) in gdbarch.sh

Fix all instances of:

    In gdbarch.sh line 2195:
            printf "            `echo "$function" | sed -e 's/./ /g'`  %s %s)\n" "$returntype" "$function"
                                ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

    Did you mean:
            printf "            $(echo "$function" | sed -e 's/./ /g')  %s %s)\n" "$returntype" "$function"

See here [1] for the rationale.

[1] https://github.com/koalaman/shellcheck/wiki/SC2006

gdb/ChangeLog:

	* gdbarch.sh: Use $(...) instead of `...`.