rules.mk: Use target specific toolchains.
We cannot use the same toolchain for both Broadcom and Airoha since they have different kernel headers and definitions.
This applies to all targets and the benefit of this to make the CI/CD pipelines more effecient instead of having different build jobs or having to run 'make distclean'.
Change-Id: I3b7e9e2d876f859ae6a23c0d45695a780f98be09