package/gdb: add gdb 7.7.x (7.7.1) and 7.6.x (7.6.2)

To prep for powerpc64le, we also disable gdb prior to 7.7.1 on
powerpc64le.

The default gdb on powerpc64le is set to 7.7.

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>