gdb: bump 7.9 series to 7.9.1

Note that both gdb 7.9 and 7.9.1 fail to build in a chroot without
texinfo, due to the infamous makeinfo issue. This should be fixed
separately.

[Peter: fixup white space]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>