[coreboot] crossgcc/buildgcc gdb7.2 version name change

[Alp Eren Köse]

gdb7.2 is renamed to gdb7.2a in
http://ftp.gnu.org/gnu/gdb/

so 'make crosgcc' fails with:
Failed to download gdb-7.2.tar.bz2.
make[1]: *** [build] Error 1
make: *** [crossgcc] Error 2

It seems, the gdb version in util/crossgcc/buildgcc needs to be updated?