target/mips: Make R5900 DMULT[U], DDIV[U], LL[D] and SC[D] user only

target/mips: Make R5900 DMULT[U], DDIV[U], LL[D] and SC[D] user only

The Linux kernel traps certain reserved instruction exceptions to
emulate the corresponding instructions. QEMU plays the role of the
kernel in user mode, so those traps are emulated by accepting the
instructions.

This change adds the function check_insn_opc_user_only to signal a
reserved instruction exception for flagged CPUs in QEMU system mode.

The MIPS III instructions DMULT[U], DDIV[U], LL[D] and SC[D] are not
implemented in R5900 hardware. They are trapped and emulated by the
Linux kernel and, accordingly, therefore QEMU user only instructions.

Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Fredrik Noring <noring@nocrew.org>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>