re PR go/92605 (r278509 causes/reveals issue in building go library)

PR go/92605
    runtime: declare runtime_usestackmaps in stack.c, not runtime.h
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/208161

From-SVN: r278540

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-9cc7092b84c38d77d98ed856f1f613a6ca27122d
+017830d2a4bd2efbddf5e841ba9ccff8acf9d7c8
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/runtime.h

@@ -475,7 +475,7 @@ bool scanstackwithmap(void*)
 bool doscanstack(G*, void*)
   __asm__("runtime.doscanstack");
 
-bool runtime_usestackmaps;
+extern bool runtime_usestackmaps;
 
 bool probestackmaps(void)
   __asm__("runtime.probestackmaps");

libgo/runtime/stack.c

@@ -16,6 +16,8 @@ extern void * __splitstack_find_context (void *context[10], size_t *, void **,
 
 #endif
 
+bool runtime_usestackmaps;
+
 // Calling unwind_init in doscanstack only works if it does not do a
 // tail call to doscanstack1.
 #pragma GCC optimize ("-fno-optimize-sibling-calls")