sim: drop extern C linkage from most sim interface headers

Since these headers merely have enum's, drop the extern C linkage
markings.  Helps to reduce the copy & paste spam.

include/gdb/sim-sh.h

@@ -19,10 +19,6 @@
 #if !defined (SIM_SH_H)
 #define SIM_SH_H
 
-#ifdef __cplusplus
-extern "C" { // }
-#endif
-
 /* The simulator makes use of the following register information. */
 
 enum
@@ -162,8 +158,4 @@ enum
   SIM_SH64_NR_FP_REGS = 64 /* number of floating point registers */
 };
 
-#ifdef __cplusplus
-}
-#endif
-
 #endif