i386: Add bf8 -> fp16 intrin

Since BF8 and FP16 have same bits for exponent, the type conversion
between them is just a cast for fraction part. We will use a sequence
of instrctions instead of new instructions to do that. For convenience,
intrins are also provided.

gcc/ChangeLog:

	* config/i386/avx10_2-512convertintrin.h
	(_mm512_cvtpbf8_ph): New.
	(_mm512_mask_cvtpbf8_ph): Ditto.
	(_mm512_maskz_cvtpbf8_ph): Ditto.
	* config/i386/avx10_2convertintrin.h
	(_mm_cvtpbf8_ph): Ditto.
	(_mm_mask_cvtpbf8_ph): Ditto.
	(_mm_maskz_cvtpbf8_ph): Ditto.
	(_mm256_cvtpbf8_ph): Ditto.
	(_mm256_mask_cvtpbf8_ph): Ditto.
	(_mm256_maskz_cvtpbf8_ph): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx10_2-512-convert-1.c: Add tests for new
	intrin.
	* gcc.target/i386/avx10_2-convert-1.c: Ditto.