[AArch64] Fix vqtb[lx][234] on big-endian

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Hi,

As mentioned by James a few weeks ago, the vqtbl[lx][234] intrinsics
are failing on aarch64_be.

The attached patch fixes them, and rewrites them using new builtins
instead of inline assembly.

I wondered about the names of the new builtins, I hope I got them
right: qtbl3, qtbl4, qtbx3, qtbx4 with v8qi and v16qi modes.

I have modified the existing aarch64_tbl3v8qi and aarch64_tbx4v8qi to
use <mode> and share the code with the v16qi variants.

In arm_neon.h, I moved the rewritten intrinsics to the bottom of the
file, in alphabetical order, although the comment says "Start of
optimal implementations in approved order": the previous ones really
seem to be in alphabetical order.

And I added a new testcase, skipped for arm* targets.

This has been tested on aarch64-none-elf and aarch64_be-none-elf
targets, using the Foundation model.

OK?

Christophe.

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Attachment: vqtbx.patch.txt
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