rs6000: Rename instruction xvcvbf16sp to xvcvbf16spn

rs6000: Rename instruction xvcvbf16sp to xvcvbf16spn

The xvcvbf16sp mnemonic, which was just added in ISA 3.1 has been renamed
to xvcvbf16spn, to make it consistent with the other non-signaling conversion
instructions which all end with "n".  The only use of this instruction is in
an MMA conversion built-in function, so there is little to no compatibility
issue.

gcc/
	* config/rs6000/rs6000-builtin.def (BU_VSX_1): Rename xvcvbf16sp to
	xvcvbf16spn.
	* config/rs6000/rs6000-call.c (builtin_function_type): Likewise.
	* config/rs6000/vsx.md: Likewise.
	* doc/extend.texi: Likewise.

gcc/testsuite/
	* gcc.target/powerpc/mma-builtin-3.c: Rename xvcvbf16sp to xvcvbf16spn.