runtime: require gdb version 7.9 for gdb test

runtime: require gdb version 7.9 for gdb test

Issue 11214 reports problems with older versions of gdb.  It does work
with gdb 7.9 on my Ubuntu Trusty system, so take that as the minimum
required version.

Fixes #11214.

Change-Id: I61b732895506575be7af595f81fc1bcf696f58c2
Reviewed-on: https://go-review.googlesource.com/12626
Reviewed-by: Austin Clements <austin@google.com>