[CI20] Call _flush_cache directly for MIPS

This is a workaround for issues with currently used toolchains:
LLVM toolchain used in Cobalt is missing compiler-rt builtins
and GCC 4.9 does not support __has_builtin nor __clear_cache.
This change adds direct call to _flush_cache().

Change-Id: Ie4dc2e4d4f2183074fe1b3d31ab3285cc74cc53c