aarch64: warn on unpredictable results for new rcpc3 instructions

[Jan Beulich]

Matthieu, Richard,

according to the documentation STILP's constraints match those of STP.
Yet gas 2.43 warns on the former but not on the latter for e.g.

	.text
	.arch armv9.4-a
	.arch_extension rcpc3
stp:
	stp	w0, w0, [x0]
	stp	x0, x0, [sp]
	stp	x0, x0, [sp, #-16]!

stilp:
	stilp	w0, w0, [x0]
	stilp	x0, x0, [sp]
	stilp	x0, x0, [sp, #-16]!

(Note also how it says "load pair" despite these being stores.)

Considering what the spec says, is there actually a need to have
separate code for the LDIAPP / STILP cases (the former being mentioned
to match LDP)?

Jan