Retire CLANG_NO_IAS34

CLANG_NO_IAS34 was introduced in r276696 to allow then-HEAD kernels to
be built with clang 3.4 in FreeBSD 10. As FreeBSD 11 and later includes
a version of Clang with a sufficiently capable integrated assembler we
do not need the workaround any longer.

Sponsored by: The FreeBSD Foundation