Added ability to accumulate in FP16 for GEMM for RISC-V

[ChipKerchner]

Added ability to accumulate in FP16 for GEMM. Widens once at the end of loops.

Testing LLVM FP16 LMUL1 VLEN256 GEMM 1 0 0  512  512  512   1  2.0  1.0  1

Total time =         24948910

Testing LLVM FP16_N LMUL1 VLEN256 GEMM 1 0 0  512  512  512   1  2.0  1.0  1

Total time =         18968190

Accumulation differences are about 4X the widening (previous) version. But the performance it up to 2.7X faster - Note: BananaPi shows only 1.85X faster.

[ChipKerchner]

Unfortunately BF16 only has widening MADD instructions. So the same changes can not be made for BF16.

[ChipKerchner]

These are for VLEN = 256 only currently

[ChipKerchner]

It now works for VLEN = 128.

[ChipKerchner]

Main loop now uses LMUL = 2

[ChipKerchner]

Even faster!!!

Testing LLVM FP16_N LMUL1 VLEN256 GEMM 1 0 0  512  512  512   1  2.0  1.0  1

Total time =         13400067

[ChipKerchner]

Convert inputs from BF16 to FP32 and use FP32 vector madds. 18% faster.