unix/main: Make static variable that's potentially clobbered by longjmp.

This fixes `error: variable 'subpkg_tried' might be clobbered by 'longjmp'
or 'vfork' [-Werror=clobbered]` when compiling on ppc64le and aarch64 (and
possibly other architectures/toolchains).

ports/unix/main.c

@@ -596,7 +596,12 @@ MP_NOINLINE int main_(int argc, char **argv) {
 
                 mp_obj_t mod;
                 nlr_buf_t nlr;
-                bool subpkg_tried = false;
+
+                // Allocating subpkg_tried on the stack can lead to compiler warnings about this
+                // variable being clobbered when nlr is implemented using setjmp/longjmp.  Its
+                // value must be preserved across calls to setjmp/longjmp.
+                static bool subpkg_tried;
+                subpkg_tried = false;
 
             reimport:
                 if (nlr_push(&nlr) == 0) {