RISC-V

[SSE4]

add some basic RISC-V support to the default conan settings.

I've tested a few popular recipes of different build systems, e.g.:

• custom (OpenSSL)
• CMake (JsonCpp, GTest)
• AutoTools (libxml2, libiconv)
• Meson (pixman)

and all were built successfully with no issues.

Changelog: Feature: Added riscv64, riscv32 architectures to default settings.yml and management of them in Meson and Autotools.
Docs: omit

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

[memsharded]

Thanks for your contribution.

Isn't it missing some declarations in settings.yml?

arch: [x86, x86_64, ppc32be, ppc32, ppc64le, ppc64,
       armv4, armv4i, armv5el, armv5hf, armv6, armv7, armv7hf, armv7s, armv7k, armv8, armv8_32, armv8.3, arm64ec,
       sparc, sparcv9,
       mips, mips64, avr, s390, s390x, asm.js, wasm, sh4le,
       e2k-v2, e2k-v3, e2k-v4, e2k-v5, e2k-v6, e2k-v7,
       xtensalx6, xtensalx106, xtensalx7]

Without this, this would probably fail

[SSE4]

Isn't it missing some declarations in settings.yml?

I think that line: https://github.com/conan-io/conan/pull/15053/files#diff-d9e6bd075cc423e1741e5ccd98478ecad42edc871876d02f63e2965c3ef07cbeR80

[memsharded]

I think that line: https://github.com/conan-io/conan/pull/15053/files#diff-d9e6bd075cc423e1741e5ccd98478ecad42edc871876d02f63e2965c3ef07cbeR80

You are right, I skipped it