[compiler-rt][builtins][RISCV] Set COMPILER_RT_HAS_FLOAT16 for RISC-V compiler-rt...

[compiler-rt][builtins][RISCV] Set COMPILER_RT_HAS_FLOAT16 for RISC-V compiler-rt tests, fixes test__extendhfsf2

Since D92241, compiler-rt/cmake/builtin-config-ix.cmake automatically tests
the host compiler for support of _Float16 and conditionally defines
COMPILER_RT_HAS_FLOAT16. That defines the macro while the compiler-rt
builtins are being built. To also define it during the compiler-rt test
runs requires whitelisting the architecture in
compiler-rt/test/builtins/CMakeLists.txt, as done in this patch. That seems
brittle. Ideally, we'd move to a solution where the target compiler was
automatically tested as well, but I'm not sure how feasible that is with the
current CMake setup.

For now, this patch whitelists RISC-V, fixing errors in test__extendhfsf2.
Alternate solutions that fix the root issue are welcome, though.

Differential Revision: https://reviews.llvm.org/D129432