put main at beginning of binary on older compilers

Reported-by: David Halls (via github)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/main.c b/main.c
index 86bd3fd..b376c0d 100644
--- a/main.c
+++ b/main.c
@@ -78,7 +78,7 @@
 	return boot_from_cbfs(base, sz);
 }
 
-int main(void)
+int __attribute__ ((section (".text.startup"))) main(void)
 {
 	setup_hw();