bitbake.conf/classes/gcc: Don't hardcode -nativesdk

bitbake.conf/classes/gcc: Don't hardcode -nativesdk

Hardcoding -nativesdk as the sdk package architecture is inflexible. We may have
multiple different target OS and we need a way to be able to separate them. Turning
this into a configurable value allows the flexibility we need to build different
SDKMACHINEs with different OS targets.

The commit should have no behaviour change, just makes things more configurable.

(From OE-Core rev: a2110e86b98d646e136de9ec6b8e668079b0d4f4)

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