target-mips: add ERETNC instruction and Config5.LLB bit

ERETNC is identical to ERET except that an ERETNC will not clear the LLbit
that is set by execution of an LL instruction, and thus when placed between
an LL and SC sequence, will never cause the SC to fail.

Presence of ERETNC is denoted by the Config5.LLB.

Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>

target-mips/cpu.h

@@ -466,6 +466,7 @@ struct CPUMIPSState {
 #define CP0C5_UFE        9
 #define CP0C5_FRE        8
 #define CP0C5_SBRI       6
+#define CP0C5_LLB        4
 #define CP0C5_UFR        2
 #define CP0C5_NFExists   0
     int32_t CP0_Config6;