Ensure GCC choice is respected by sdc.mak makefile
Ensure GCC choice is respected by sdc.mak makefile

This is the only place in any of the makefiles where `gcc` was invoked
directly, so this tweak should ensure that when the user tries to build
with `make GCC=...` their choice will indeed be used.

SDC-Developers/SDC,deadalnix/SDC,UplinkCoder/SDC,SDC-Developers/SDC,SDC-Developers/SDC,UplinkCoder/SDC,UplinkCoder/SDC,deadalnix/SDC,deadalnix/SDC