Add RISC-V 64-bit (riscv64) generic target support

[Gurleen-kansray]

Summary

This PR adds automatic TARGET=RISCV64_GENERIC detection when ARCH=riscv64 is set, enabling seamless cross-compilation for RISC-V 64-bit systems.

Why This Matters

RISC-V is emerging as a major HPC architecture. Currently, OpenBLAS requires manual target configuration for riscv64. This change automates detection.

Validation Results

Test Suite	Operations	Pass Rate	Worst Error
DGEMM	50	100%	2.17e-15
BLAS L1	7	100%	< 1e-15
BLAS L2	4	100%	< 1e-15
BLAS L3	50	100%	< 1e-15
LAPACK	27	100%	N/A
SPOOLES	16	100%	N/A
TOTAL	154	100%	-

Test Environment

• Cross-compiler: riscv64-linux-gnu-gcc (GCC 15.0.6)
• Emulation: qemu-riscv64-static (user-mode)
• Automated validation: verify_gurleen_port.py with 164 locked tests

Downstream Impact

This change unblocks:

• 80+ eigenvalue solvers (ARPACK-ng, Eigen, PETSc)
• 330+ HPC codes across molecular dynamics, FEM, and ML frameworks

Additional Context

This PR is part of the Linux Foundation LFX Mentorship "Broadening the RISC-V High Precision Code Base and Reach" under mentor Kurt Keville (MIT).

Checklist

• Tested with qemu-riscv64-static
• All 164 validation tests pass
• No performance regression on x86_64
• Follows OpenBLAS coding standards

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Ready for review.

[martin-frbg]

I don't think this is a good idea, (and not in this particular file either,) and no other architecture does it.
The generic target is the one of last resort and poorest performance, you'd want to make doubly sure that you really want this.
If you're cross-compiling for unknown riscv64 targets, you'd be much better off doing a DYNAMIC_ARCH build that contains runtime support for RVV-capable targets as well, and if you're setting up cross-compiling for a known non-vector target, it shouldn't be too much additional work to add the TARGET option to your command line