[libpng18] reworked SIMD code

[jbowler]

This is the rework of the hardware (SIMD) support based on @csparker247's
pull request #575.

It incorporates that request rebased on the current master; the request
should be pulled first.

This passes cross and hosted build checks as follows:

aarch64 configure make all SUCCESS
aarch64 cmake make all SUCCESS
amd64 configure make all SUCCESS
amd64 cmake make all SUCCESS
armv7a configure make all SUCCESS
armv7a cmake make all SUCCESS
armv7a-neon configure make all SUCCESS
armv7a-neon cmake make all SUCCESS
clang cmake make all SUCCESS
mips configure make all SUCCESS
mips cmake make all SUCCESS
riscv64 configure make all SUCCESS
riscv64 cmake make all SUCCESS
clang configure make check SUCCESS
clang cmake make test SUCCESS

• cmake: Remove architecture-specific checks for hardware optimization
• Full compile time support for target-specific code
• MIPS support clean-up

[jbowler]

I've squashed the changes into one commit and removed the file moves. Firstly there is a good chance of merge conflicts with file moves and the merge resolution normally ends up dropping changes accidentally. Secondly I'm not sure they were correct.

What I favor, after all the pending pull requests have been resolved, is a directory tree structure where all the target-specific code is in one directory. Possibly a subdirectory called "arch" with subdirectories below that for each individual architecture (i.e the first part of a CHOST tuple). This kind of structure:

arch/<architecture>/<instruction set>/<implementation>

Where the instruction set subdirectory is only necessary if there are multiple implementations for different instruction sets (as on MIPS). In fact it doesn't matter much below the architecture level because my idea is that third party maintainers store third party code in there (licensed under the libpng license of course) and never alter anything above that level. When the implementation is satisfactory the libpng maintainer adds the one required line to pngtarget.h as in the current code; no changes are required (or permitted :-) in pngsimd.c

[jbowler]

The Lint changes did not pick up the errors detected by editorconfig-checker. I had to run editorconfig-checker directly to detect them.. Something in the recent changes (today) has broken the cmakeAMD64 builds (make check/test).

[jbowler]

No; it's the 10700 problem, it was always there in any make check run.

[jbowler]

Rebased on #596. Make check and make test now pass on AMD64, ararch64, armv7a (with and without -mfpu=neon), mips and riscv64 (no target specific code for that yet) all now build. aarch64 and armv7a were passing the tests too insofar as I could run them (a couple of days ago, before the latest merge). MIPS32 builds (no target specific code) but I can't manage to make a MIPS64 compiler; all my crossdevs fail at the last stage of building gcc.

clang was tested too on AMD64 (making clang cross compilers appears to be a little more difficult.)

[jbowler]

Rebased, all tests passed

[jbowler]

This should wait because it is development rather than a bug fix. It's not clear how that (development) will be done at this point.