base.bbclass: Deferred inherit native toolchain class

base.bbclass: Deferred inherit native toolchain class

use TOOLCHAIN_NATIVE variable for selecting native compiler

Default it to PREFERRED_TOOLCHAIN_NATIVE, a recipe which wants
to enforce a toolchain can do so with

for cross toolchains ( e.g. target, nativesdk )

TOOLCHAIN = "gcc"

For native

TOOLCHAIN_NATIVE = "gcc"

This helps build native recipe with clang as native compiler.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>