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

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

Current trick to pick up GNU assembler minor version does not work with the
latest binutils (2007-03-01 or later) due to ${PKGVERSION} now default to
"(GNU Binutils) ".

  $ sde-as --version |grep "GNU assembler"
  GNU assembler 2.15.94 mipssde-6.02.02-20050602
  $ sde-as --version |grep "GNU assembler" |awk '{print $3}'
  2.15.94
  $ sde-as --version |grep "GNU assembler" |awk '{print $3}' |awk -F. '{print $2}'
  15
  $

  $ mips-linux-as --version |grep "GNU assembler"
  GNU assembler (GNU Binutils) 2.18
  $ mips-linux-as --version |grep "GNU assembler" |awk '{print $3}'
  (GNU
  $ mips-linux-as --version |grep "GNU assembler" |awk '{print $3}' |awk -F. '{print $2}'
  (no output)
  $

As a result of above, you'll see many noises with such binutils:

  make -C cpu/mips/
  /bin/sh: line 0: [: : integer expression expected
  /bin/sh: line 0: [: : integer expression expected
  make[1]: Entering directory `/home/skuribay/devel/u-boot.git/cpu/mips'
  mips-linux-gcc  -D__ASSEMBLY__ -g  -Os   -D__KERNEL__ -DTEXT_BASE=0xB0000000  -I/home/skuribay/devel/u-boot.git/include -fno-builtin -ffreestanding -nostdinc -isystem /home/skuribay/devel/buildroot/build_mips/staging_dir/usr/bin/../lib/gcc/mips-linux-uclibc/4.2.1/include -pipe  -DCONFIG_MIPS -D__MIPS__ -G 0 -mabicalls -fpic -pipe -msoft-float -march=4kc -mtune=4kc -EB -c -o incaip_wdt.o incaip_wdt.S
  /bin/sh: line 0: [: : integer expression expected
  mips-linux-gcc  -D__ASSEMBLY__ -g  -Os   -D__KERNEL__ -DTEXT_BASE=0xB0000000  -I/home/skuribay/devel/u-boot.git/include -fno-builtin -ffreestanding -nostdinc -isystem /home/skuribay/devel/buildroot/build_mips/staging_dir/usr/bin/../lib/gcc/mips-linux-uclibc/4.2.1/include -pipe  -DCONFIG_MIPS -D__MIPS__ -G 0 -mabicalls -fpic -pipe -msoft-float -march=4kc -mtune=4kc -EB -c -o cache.o cache.S
  /bin/sh: line 0: [: : integer expression expected
  mips-linux-gcc -g  -Os   -D__KERNEL__ -DTEXT_BASE=0xB0000000  -I/home/skuribay/devel/u-boot.git/include -fno-builtin -ffreestanding -nostdinc -isystem /home/skuribay/devel/buildroot/build_mips/staging_dir/usr/bin/../lib/gcc/mips-linux-uclibc/4.2.1/include -pipe  -DCONFIG_MIPS -D__MIPS__ -G 0 -mabicalls -fpic -pipe -msoft-float -march=4kc -mtune=4kc -EB -Wall -Wstrict-prototypes -c -o asc_serial.o asc_serial.c
  /bin/sh: line 0: [: : integer expression expected

This patch simplifies the trick and makes it work with both versions of gas.
I also replace an expensive `awk (or gawk)' with `cut'.

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