gdb: fix shellcheck warnings SC2086 (missing double quotes) in gdbarch.sh

gdb: fix shellcheck warnings SC2086 (missing double quotes) in gdbarch.sh

Fix all instances of:

    In gdbarch.sh line 31:
        if test ! -r ${file}
                     ^-----^ SC2086: Double quote to prevent globbing and word splitting.

    Did you mean:
        if test ! -r "${file}"

Note that some instances of these are in text that is eval'ed.  I'm
pretty sure that things could go wrong during the eval too, but that's
not something shellcheck can check.

gdb/ChangeLog:

	* gdbarch.sh: Use double quotes around variables.