buildgcc: Break if parts of the toolchain are missing

We test for the presence of a couple of tools and even print an error.
But the tool didn't stop there.

Change-Id: I40dcf7894408ea7b24d5f68c76df4b7541f469bd
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/111
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>