gdb: fix missing operator % on xmethod matcher output

Fixed missing operator % on xmethod matcher registration output and, as
suggested on bug 32532, converted both uses of operator % to str.format.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32352
Change-Id: Ic471516292c2f1d6d1284aaeaea3ec14421decb8
1 file changed