sim: rl78: move storage out of header

This port declares its pc variable in a header and then includes
it multiple times.  This causes linker errors with newer gcc due
to the change in -fno-common behavior.  Move the storage to a C
file so we only have one instance of it in the final program.

sim/rl78/cpu.c

@@ -36,6 +36,8 @@ int rl78_g10_mode = 0;
 int g13_multiply = 0;
 int g14_multiply = 0;
 
+SI pc;
+
 #define REGISTER_ADDRESS 0xffee0
 
 typedef struct {