Fix issue #21500 : [importc] undefined reference __Float32x4_t

[thewilsonator]

TODO: tests

NB, this should fix the undefined identifier.
This does not address __attribute__ ((__aarch64_vector_pcs__)) which I presume would need to be a calling convention in LDC.

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[kassane]

Please, check #21500 (comment)

[kinke]

This was already fixed (properly) in #21772:

dmd/druntime/src/__importc_builtins.di

Lines 118 to 123 in f18ac3e

	version (CRuntime_Glibc) version (AArch64)
	{
	// math.h needs these
	alias __Float32x4_t = __vector(float[4]);
	alias __Float64x2_t = __vector(double[2]);
	}

[ibuclaw]

.