gdb: fix shellcheck warnings SC2059 (variables in printf format string) in gdbarch.sh

gdb: fix shellcheck warnings SC2059 (variables in printf format string) in gdbarch.sh

Fix all instances of this:

    In gdbarch.sh line 2182:
                printf "  gdb_assert (!(${invalid_p}));\n"
                       ^-- SC2059: Don't use variables in the printf format string. Use printf "..%s.." "$foo".

... by doing exactly as the message suggests.

The rationale explained here [1] makes sense, if there happens to be a
format specifier in text substituted for the variable, the printf won't
do what we expect.

[1] https://github.com/koalaman/shellcheck/wiki/SC2059

gdb/ChangeLog:

	* gdbarch.sh: Use %s with printf, instead of variables in the
	format string.