sim: mips: avoid _ namespace

Some C libraries export _P symbols in their headers (like older
newlib and its ctype.h), so use P_ instead to avoid conflicts.

sim/mips/interp.c

diff --git a/sim/mips/interp.c b/sim/mips/interp.c
index 30d417b..2cc7524 100644
--- a/sim/mips/interp.c
+++ b/sim/mips/interp.c
@@ -1468,14 +1468,14 @@
 			  sim_io_printf(sd,"<binary not supported>");
 			else
 			  {
-#define _P(c, fmt64, fmt32) \
+#define P_(c, fmt64, fmt32) \
   case c: \
     if (longlong) \
       sim_io_printf (sd, "%" fmt64, lv); \
     else \
       sim_io_printf (sd, "%" fmt32, (int)lv); \
     break;
-#define P(c, fmtc) _P(c, PRI##fmtc##64, PRI##fmtc##32)
+#define P(c, fmtc) P_(c, PRI##fmtc##64, PRI##fmtc##32)
 			    switch (c)
 			      {
 			      P('d', d)
@@ -1486,7 +1486,7 @@
 			      }
 			  }
 #undef P
-#undef _P
+#undef P_
 		      }
 		    else if (strchr ("eEfgG", c))
 		      {