[MIPS] cpu/mips/config.mk: Fix GNU assembler minor version picker

Current trick to pick up GNU assembler minor version uses a dot(.) as a
delimiter, and take the second field to obtain minor version number. But
as can be expected, this doesn't work with a version string which has
dots more than needs.

Here's an example:

$ mips-linux-gnu-as --version | grep 'GNU assembler'
GNU assembler (Sourcery G++ Lite 4.2-129) 2.18.50.20080215
$ mips-linux-gnu-as --version | grep 'GNU assembler' | cut -d. -f2
2-129) 2
$

This patch restricts the version format to 2.XX.XX... This will work
in most cases.

$ mips-linux-gnu-as --version | grep 'GNU assembler' | egrep -o '2\.[0-9\.]+'
2.18.50.20080215
$ mips-linux-gnu-as --version | grep 'GNU assembler' | egrep -o '2\.[0-9\.]+' | cut -d. -f2
18
$

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>