[U-Boot] standalone-examples: support custom GCC lib

Fighting with compiling u-boot today and dealing with problems related to using a custom built openembedded toolchain, I found the need to specify a custom location for the GCC lib.

This is well supported when building u-boot, but my compile was failing in the standalone examples, as there was no support for a custom GCC lib in the standalone examples Makefile.

I am no toolchain or u-boot expert, and as such to avoid the firey pits of flamewar hell, this is an RFC ;)