Wrap new Clang builtin __builtin_cheri_clear_cap_tag() with a more
programmer-friendly cheri_ccleartag() macro for use in CheriBSD.
Add a test_fault_tag() to cheritest to exercise an untagged capability
indirection outside of a sandbox.
Prefer cheri_ccleartag() to create an untagged but otherwise valid
capability for the in-sandbox tag violation test in cheritest-helper.