src/cmd/internal/obj/riscv: reuse high offset value when accessing stack

[newborn22]

Related Issue(s) & Descriptions

The background has been described in golang#79298 and corresponding PR is submitted in golang#79299.

Performance data

goos: linux
goarch: riscv64
pkg: github.com/tetratelabs/wazero/internal/integration_test/bench
cpu: SG2044
                                                   │ wazero_base.txt │        wazero_stack_opt.txt         │
                                                   │     sec/op      │   sec/op     vs base                │
Invocation/interpreter/fib_for_20                       10.660m ± 4%   8.456m ± 7%  -20.67% (p=0.000 n=10)
Invocation/interpreter/string_manipulation_size_50       4.125m ± 5%   3.630m ± 5%  -12.00% (p=0.000 n=10)
Invocation/interpreter/random_mat_mul_size_20            27.02m ± 7%   22.55m ± 1%  -16.55% (p=0.000 n=10)
geomean                                                  10.59m        8.846m       -16.48%

Checklist

• Tests were added or are not required
• Documentation was added or is not required