Wrapper for GCC that uses liblinux

[penberg]

Add a GCC wrapper, similar to musl-gcc, which overrides the standard library with liblinux and libmanticore. The way to do this is with GCC spec files:

https://gcc.gnu.org/onlinedocs/gcc/Spec-Files.html

The benefit of this wrapper is that application porting is easier because only CC needs to be overridden for Manticore configuration (in theory anyway).