Kbuild: LDLIBS_TOOLCHAIN adds toolchain static libraries.

This commit adds the variable LDLIBS_TOOLCHAIN that, by default,
add the gcc static library to the link command.
This variable can be overriden as an environmental variable or as a make
parameter to allow the use of different toolchains different from gcc.

Change-Id: Id9d6c8621a7dc8feebdf377c115564701fb7652b
Signed-off-by: Juan Manuel Cruz <juan.m.cruz.alcaraz@linux.intel.com>