fix silly portability bug

R=gri
OCL=15238
CL=15238

src/runtime/rt1_amd64_darwin.c

@@ -6,6 +6,9 @@
 #include "amd64_darwin.h"
 #include "signals.h"
 
+extern void _rt0_amd64_darwin();
+byte* startsym = (byte*)_rt0_amd64_darwin;
+
 typedef uint64 __uint64_t;
 
 /* From /usr/include/mach/i386/_structs.h */

src/runtime/rt1_amd64_linux.c

@@ -6,6 +6,9 @@
 #include "amd64_linux.h"
 #include "signals.h"
 
+extern void _rt0_amd64_linux();
+byte* startsym = (byte*)_rt0_amd64_linux;
+
 /* From /usr/include/asm-x86_64/sigcontext.h */
 struct _fpstate {
   uint16   cwd;

src/runtime/rt2_amd64.c

@@ -94,11 +94,10 @@ int32
 inlinetrap(int32 sig, byte* pc)
 {
 	extern void etext();
-	extern void _rt0_amd64_darwin();
 
 	if(sig != 5)	/* SIGTRAP */
 		return 0;
-	if(pc-2 < (byte*)_rt0_amd64_darwin || pc >= (byte*)etext)
+	if(pc-2 < startsym || pc >= (byte*)etext)
 		return 0;
 	if(pc[-2] != 0xcd)  /* INTB */
 		return 0;

src/runtime/runtime.h

@@ -208,6 +208,7 @@ G*	allg;
 int32	goidgen;
 extern	int32	gomaxprocs;
 extern	int32	panicking;
+extern	byte	*startsym;
 
 /*
  * common functions and data