Bug fix: cannot convert 'const short int*' to 'const __bf16*'

Bug fix: cannot convert 'const short int*' to 'const __bf16*'

This patch fixes a bug introduced by my earlier patch (
https://gcc.gnu.org/pipermail/gcc-patches/2020-March/541680.html ).
It introduces a new scalar builtin type that was missing in the original
patch.

Bootstrapped cleanly on arm-none-linux-gnueabihf.
Tested for regression on arm-none-linux-gnueabihf. No regression from
before the original patch.
Tests that failed or became unsupported because of the original tests
now work as they did before it.

	* config/arm/arm-builtins.c
	(arm_init_simd_builtin_scalar_types): New.
	* config/arm/arm_neon.h (vld2_bf16): Used new builtin type.
	(vld2q_bf16): Used new builtin type.
	(vld3_bf16): Used new builtin type.
	(vld3q_bf16): Used new builtin type.
	(vld4_bf16): Used new builtin type.
	(vld4q_bf16): Used new builtin type.
	(vld2_dup_bf16): Used new builtin type.
	(vld2q_dup_bf16): Used new builtin type.
	(vld3_dup_bf16): Used new builtin type.
	(vld3q_dup_bf16): Used new builtin type.
	(vld4_dup_bf16): Used new builtin type.
	(vld4q_dup_bf16): Used new builtin type.