[VE][compiler-rt] Support VE in clear_cache.c

Support SX Aurora VE by __clear_cache() function.  This modification
allows VE to run written data, e.g. clear_cache_test.c under compiler-rt
test.  We still have code alignment problem in enable_execute_stack_test.c,
though.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D92703