Makefile: Initial support for using CS cache for CC

Codebench Sourcery toolchains are capable of using their own method
of compile caching. This patch adds initial support by allowing the
capability of using the -eabi triplet after setting PATH to link both
the cache & cc bins.

Usecase example:

  export PATH=/path-to-toolchain/bin/cache:/path-to-toolchain/bin:$PATH
  export CS_GCC_TRIPLET=arm-none-eabi-

This will probably end up in a script somewhere and probably isn't the
ideal way to implemenet this, but it is the simplest & most immediate
way I could think of.

Using a slow dual core laptop with 3GB of RAM to compile this kernel (-j4):
(results using the default -O2 aren't far off)

gcc-4.7:
	-Ofast:
		Build One -
		real	32m7.383s
		user	45m34.239s
		sys	6m9.675s

		Build Two -
		real	7m20.906s
		user	6m16.320s
		sys	2m2.368s

		Build Three -
		real	6m38.865s
		user	6m2.159s
		sys	1m58.403s

Signed-off-by: Luis Cruz <ljc2491@gmail.com>