[RISCVISelLowering] avoid emitting libcalls to __mulodi4() and __multi3()

Similar to D108842, D108844, D108926, D108928, and D108936.

__has_builtin(builtin_mul_overflow) returns true for 32b RISCV targets,
but Clang is deferring to compiler RT when encountering long long types.

If the semantics of __has_builtin mean "the compiler resolves these,
always" then we shouldn't conditionally emit a libcall.

Link: https://bugs.llvm.org/show_bug.cgi?id=28629

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D108939