Support new dual-linker option.

Let build automatically detect binutils versions and append proper
--enable-gold=xxx options to binutils configuration.
The story is that gcc-4.6 and binutils-2.21 use
--enable-gold=default, our old gcc and binutils use
--enable-gold=both/gold.
With the patch, we pass toolchain configuration --enable-gold=xxx to
gcc, and let build script automatically detect binutils version
number and append proper --enable-gold=xxx to binutils configuration.

Change-Id: Iaf31e3285107b3afb7a2f0a66c58b02a9937e214