rvjit: compute mulhsu sign correction before clobbering rs1

[carlosqwqqwq]

rvjit: compute mulhsu sign correction before clobbering rs1

Fixes #265

Commit message

rvjit: compute mulhsu sign correction before clobbering rs1

Description

The x86 JIT computes the unsigned high product before the sign-correction term. When rd == rs1, that product overwrites the source still needed by the correction. Compute the correction first, then emit the high product and add it back. This is separate from the division overflow guard.

Validation

• Native RV64 hardware and QEMU provide the reference result.
• JIT and interpreter controls isolate the aliasing case.
• The proposed change is limited to src/rvjit/rvjit_x86.h::rvjit_x86_mulhsu().
• Fixed and negative controls were verified against the references.