[clang][RISCV] Set HasLegalHalfType to true if zfh is enabled

The desired semantics for HasLegalHalfType are slightly unclear in that
the comment for HasLegalHalfType says "True if the backend supports
operations on the half LLVM IR type." Which operations? We get very
limited scalar operations with zfhmin, more with zfh, and vector support
with zvfh. While the comment for hasLegalHalfType() says "Determine
whether _Float16 is supported on this target."

This patch sets HasLegalHalfType to true for zfh.

Differential Revision: https://reviews.llvm.org/D145071