Revert submission 2221893-silvermont-toolchain
Reason for revert: broke aosp_master build at b/249495321
Reverted Changes:
If32dbdab5:run bp2build in mixed_mode_test.sh
Ia96a2a82c:create cc_register_toolchains function
I074915cfa:create cc_register_toolchains function
Change-Id: I79717a23ebe2d7fe00b1b288acc96ce147e6ae92