arc: move disabling/invalidation of caches earlier

Too much code might run with a stale icache, before _PrepC(), so move
the invalidation/disabling code earlier. The asm code does the exact
same thing disable_icache() and invalidate_dcache() were doing.

Change-Id: If52f4e4a1de546fb82873c91ead95614a44b106d
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>