Change make crossgcc to build without gdb by default

Using gdb with coreboot is not (yet) very common, so at least for
now it makes sense to not build gdb by default. A make crosstools
target is also added, which runs the full build in util/crossgcc
and thus generates a toolchain with both compiler and debugger.

Change-Id: I939ebcd06ae9a1bc485fd18e70cac98112d3bbbf
Signed-off-by: Peter Stuge <peter@stuge.se>
Reviewed-on: http://review.coreboot.org/17
Tested-by: build bot (Jenkins)
Reviewed-by: Marshall Buschman <mbuschman@lucidmachines.com>