Avoid startup section overflow with pure-core or no optimize compile

teensy3/mk20dx128.c

 // Clang seems to generate slightly larger code with Os than gcc
 __attribute__ ((optimize("-Os")))
 #else
+#if defined(__PURE_CODE__) || !defined(__OPTIMIZE__)
+// cases known to compile too large for 0-0x400 memory region
+__attribute__ ((optimize("-Os")))
+#else
+// hopefully all others fit into startup section (below 0x400)
 __attribute__ ((section(".startup"),optimize("-Os")))
 #endif
+#endif
 void ResetHandler(void)
 {
 	uint32_t *src = &_etext;