This patch implements the __jcvt ACLE intrinsic [1] that maps down to the FJCVTZS [2] instruction from Armv8.3-a.
No fancy mode iterators or nothing. Just a single builtin, UNSPEC and define_insn and the associate plumbing.
This patch also defines __ARM_FEATURE_JCVT to indicate when the intrinsic is available.