build: Update benchmark submodule for RISC-V

[wanghan-sanechips]

Fixes RISC-V Illegal Instruction Error

• Pins to b20cea67 to fix illegal instruction
• Verified on openEuler RISC-V

Problem

snappy_benchmark crashes on RISC-V platforms due to:

• Use of privileged rdcycle instruction in older Google Benchmark versions
• Linux 6.6+ blocks rdcycle in userspace

Solution

Update benchmark submodule to b20cea67 which:

• Switches to rdtime instruction (userspace-safe)
• Maintains backward compatibility
• Passes all tests on RISC-V

Testing

Environment	Result
Hardware	SG2042
OS	openEuler 25.03
Kernel Version	Linux openeuler-riscv64 6.6.0-72.6.0.56.oe2503.riscv64
Benchmark Result	All tests passed

[google-cla]

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[wanghan-sanechips]

Hi @haney , this is my first contribution to the project. Could you please approve running the CI workflow? Thank you!