Add githash to binutils version string.

So the output for 'ld -v' is something like -
GNU ld (binutils-2.25-3febaa7) 2.25.51.20141117

Tested by: ./build.py --toolchain arm-linux-androideabi --host linux

Change-Id: I4687fb5696bf3aa7a08a94149090055a76ec2294