Previously, sha-x86_64.pl (was sha512-x86_64.pl) was used to generate
both sha256-x86_64.S and sha512-x86_64.S. This required special build
logic in ring.mk and also would have required special logic in the
msbuild project files. This commit adds dedicated perlasm source files
for both, and there's now a 1-to-1 correspondence between perlasm
scripts and output files, which avoids any need for special-case logic
in the build system.
The same goes for sha-armv8.pl (was sha512-armv8.pl).
72e2935ff2