gdb: silence shellcheck warning SC2162 (use read -r) in gdbarch.sh

gdb: silence shellcheck warning SC2162 (use read -r) in gdbarch.sh

shellcheck reports:

    In gdbarch.sh line 53:
        while IFS='' read line
                     ^--^ SC2162: read without -r will mangle backslashes.

See the rationale at [1].  In our case, we actually want the backslashes
to be interpreted and removed.  Silence the warning using a directive.

[1] https://github.com/koalaman/shellcheck/wiki/SC2162

gdb/ChangeLog:

	* gdbarch.sh (do_read): Add shellcheck disable directive for
	warning SC2162.