[Rpm-maint] [rpm-software-management/rpm] RFC: Add x86-64 architecture levels (v2-v4) as architectures (PR #2315)

[Fabian Vogt]

The x86_64 SysV psABI defines four levels of x86_64 with certain CPU features required for each level. Those definitions are meant to be generically useful and recognized as such by glibc and gcc as well.

For backward-compatibility and to avoid surprises, default to building x86_64 even on v2+ capable machines.

Tested by running `rpm --eval %_target_cpu` and using `rpmbuild` with various `BuildArch` values on a `x86_64_v3` host.

This should be less controverisal than adding CPU model specific architectures such as `znver1` (https://github.com/rpm-software-management/rpm/pull/1035).

There might be one slight issue: The psABI document defines x86-64-v3 as
x86-64-v2 + AVX + AVX2 + F16C + FMA + LZCNT + MOVBE + BMI + BMI2 + OSXSAVE, but glibc's hwcaps code only looks at
x86-64-v2 + AVX + AVX2 + F16C + FMA + LZCNT + MOVBE (no BMI, BMI2 or OSXSAVE). Not sure whether that's a bug or for some reason intentional. gcc uses the psABI definition, so code built with `-march=x86-64-v3` is not compatible with `%_libdir/glibc-hwcaps/x86-64-v3/`...

CC @Conan-Kudo @dirkmueller @DimStar77 @mlschroe
You can view, comment on, or merge this pull request online at:

  https://github.com/rpm-software-management/rpm/pull/2315

-- Commit Summary --

  * Add x86-64 architecture levels (v2-v4) as architectures

-- File Changes --

    M lib/rpmrc.c (75)
    M rpmrc.in (18)

-- Patch Links --

https://github.com/rpm-software-management/rpm/pull/2315.patch
https://github.com/rpm-software-management/rpm/pull/2315.diff

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