[ihb2032]

What and why

Mirror the existing arm/aarch64/ppc target family for RISC-V 64:

• make riscvbuild — cross-build all of zstd with riscv64-linux-gnu-gcc -Werror
• make riscvfuzz — run the fuzz suite under qemu-riscv64-static
• make riscvtest — run the test suite under qemu-riscv64-static

Checklist

• I searched open pull requests and issues: this problem is not already being addressed.
• Bug fix: it comes with a reproducer or a test that fails without the fix.
• Performance change: numbers from repeated runs are included, with CPU, compiler, flags and data.
  A gain in one configuration is not a general gain: see the Performance section of CONTRIBUTING.md.
• I stated below whether AI or other automated tools were used.

Tools used

AI (glm) was used only to help identify potential issues and assist with problem investigation. All code changes, implementation decisions, and validation were performed manually.