toolchain.gccarmemb: Simplify setting of libgcc include path

Utilize the gcc -print-libgcc-file-name to determine the path to
libgcc.a.  Using this also resolve issues if we are building with
floating point and the various ABIs to pickup the right lib.

We can also simply a few Makefiles and remove the need to export
_CROSS_COMPILE_arm_isa.

Change-Id: I293394c921c18fa4984173ba51825d6f9a333697
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>