[arm64] Negative immediates and lack of default Neon support

[Torbjörn Granlund]

There seem to be another problem with using clang for arm64, see the
build failures at <https://gmplib.org/devel/tm-date.html>.

Apparently, clang disables Neon instruction assembly by default.  I
found that passing -mfpu=neon to the compiler helps.

This command line option is not recognised by gcc.  We therefore need to
split out clang handling in configure.ac.

A related problem is whether Neon is actually optional for armv8.  My
understanding is that it is not optional, but I am not sure about that.
If it is optional, then we need to move Neon assembly code into a
arm64/neon subdirectory, and adjust configure.ac accordingly.

-- 
Torbjörn
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