[BuildLibCalls][RISCV] Sign extend return value of bcmp on riscv64.

riscv64 wants callees to sign extend signed and unsigned int returns.

The caller can use this to avoid a sign extend if the result is
used by a comparison since riscv64 only has 64-bit compares.

InstCombine/SimplifyLibCalls aggressively turn memcmps that are only
used by an icmp eq 0 into bcmp, but we lose the signext attribute that
would have been present on the memcmp. This causes an unneeded sext.w
in the generated assembly.

This looks even sillier if bcmp is implemented alias to memcmp. In
that case, not only did we not get any savings by using bcmp, we added
an instruction.

This probably applies to other functions, this just happens to be
the one I noticed so far.

See also the discussion here https://discourse.llvm.org/t/can-we-preserve-signext-return-attribute-when-converting-memcmp-to-bcmp/67126

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D139901