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.