meta: Add TCOVERRIDE for toolchain selection at recipe scope

meta: Add TCOVERRIDE for toolchain selection at recipe scope

TCOVERRIDE is defined to toolchain-<TOOLCHAIN> and its added to OVERRIDES
that a recipe can see and it can use "toolchain-gcc" or "toolchain-clang"
to set specific metadata based upon global distro toolchain policy.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>