build: use gcc instead of ld for linking

We now have to supply location of libraries (libgcc, libc) and header
directories to the build system. This can be solved in the
Makefile.toolchain.* file for known toolchains but becomes a problem
when supporting 3rd party SDKs and cross compilers.

Linking with ld requires ld to know where all the artificats of the
compiler are. Use gcc instead to make use of the internal information
available in gcc about additional libraries and headers.

Jira: ZEP-241
Change-Id: I15a3b812467b7ae878226f7e5532d538d2720268
Signed-off-by: Anas Nashif <anas.nashif@intel.com>