Fix MIPS HI16 relocation overflow

[plafosse]

Summary

• mask the computed R_MIPS_HI16 value to the instruction’s 16-bit immediate field
• prevent a carry above bit 15 from corrupting LUI’s destination register

Reproduction

Loading a MIPS32 relocatable ELF at 0x1_0000_0000 produced a 17-bit high-half value (0x10000). The unmasked value changed lui $v0, 0 (0x3c020000) into lui $v1, 0 (0x3c030000).

Testing

• built the standalone arch_mips plugin successfully
• loaded the reproducer with the rebuilt plugin at 0x1_0000_0000
• verified the relocated instruction remains lui $v0, 0