arm: Fix arm backend-use of (u|s|us)dot_prod patterns

Given recent changes to the dot_prod standard pattern name, this patch
fixes the arm back-end by implementing the following changes:

1. Add 2nd mode to all patterns relating to the dot-product in .md
files.
2. redirect the single-mode CODE_FOR_neon_(u|s|us)dot<mode> values
generated from `arm_neon_builtins.def' to their new 2-mode
equivalents via means of simple aliases, as per the following example:

  constexpr insn_code CODE_FOR_neon_sdotv8qi
    = CODE_FOR_neon_sdotv2siv8qi;

gcc/ChangeLog:

	* config/arm/neon.md (<sup>dot_prod<vsi2qi>): Renamed to...
	(<sup>dot_prod<mode><vsi2qi>): ...this.
	(neon_<sup>dot<vsi2qi>): Renamed to...
	(neon_<sup>dot<mode><vsi2qi>): ...this.
	(neon_usdot<vsi2qi>): Renamed to...
	(neon_usdot<mode><vsi2qi>): ...this.
	(usdot_prod<vsi2qi>): Renamed to...
	(usdot_prod<mode><vsi2qi>): ...this.
	* config/arm/arm-builtins.cc
	(CODE_FOR_neon_sdotv8qi): Definie as alias to
	new CODE_FOR_neon_sdotv2siv8qi.
	(CODE_FOR_neon_udotv8qi): Definie as alias to
	new CODE_FOR_neon_udotv2siv8qi.
	(CODE_FOR_neon_usdotv8qi): Definie as alias to
	new CODE_FOR_neon_usdotv2siv8qi.
	(CODE_FOR_neon_sdotv16qi): Definie as alias to
	new CODE_FOR_neon_sdotv4siv16qi.
	(CODE_FOR_neon_udotv16qi): Definie as alias to
	new CODE_FOR_neon_udotv4siv16qi.
	(CODE_FOR_neon_usdotv16qi): Definie as alias to
	new CODE_FOR_neon_usdotv4siv16qi.