[AArch64] Implement __jcvt intrinsic from Armv8.3-A
The jcvt intrinsic defined in ACLE  is available when ARM_FEATURE_JCVT is defined.
This change introduces the AArch64 intrinsic, wires it up to the instruction and a new clang builtin function.
The __ARM_FEATURE_JCVT macro is now defined when an Armv8.3-A or higher target is used.
I've implemented the target detection logic in Clang so that this feature is enabled for architectures from armv8.3-a onwards (so -march=armv8.4-a also enables this, for example).
make check-all didn't show any new failures.
Differential Revision: https://reviews.llvm.org/D64495