bitbake.conf: Start to separate out gcc definition

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Author: rpurdie

meta/conf/bitbake.conf

@@ -841,7 +841,8 @@ include conf/licenses.conf
 require conf/sanity.conf
 include conf/bblock.conf
 
-require toolchain/gcc.inc
+TOOLCHAIN ??= "gcc"
+require toolchain/${TOOLCHAIN}.inc
 require toolchain/build-gcc.inc
 
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