[SYCL][LIBCLC][NATIVECPU] Fix ldexp and popcount on Native CPU

[PietroGhg]

ldexp in generic (for half) and popcount in native_cpu are implemented using clang builtins. This PR makes it so we call the correct "overload" of the builtin:

• __builtin_ldexpf16 for half
• __builtin_popcountg for unsigned integer types
• for signed integer types the implicit cast to int performed by calling __builtin_popcount doesn't work due to sign extension, so we explicitly cast to unsigned char or unsigned short, and use __builtin_popcountg.

[PietroGhg]

Hi @intel/llvm-gatekeepers, this looks ready to merge, thank you