py/mkrules.mk: Don't pass COPT to linker.

Oftentimes, libc, libm, etc. don't come compiled with CPU compressed code
option (Thumb, MIPS16, etc.), but we may still want to use such compressed
code for MicroPython itself.

py/mkrules.mk

@@ -81,7 +81,9 @@ all: $(PROG)
 
 $(PROG): $(OBJ)
 	$(ECHO) "LINK $@"
-	$(Q)$(CC) $(COPT) -o $@ $^ $(LIB) $(LDFLAGS)
+# Do not pass COPT here - it's *C* compiler optimizations. For example,
+# we may want to compile using Thumb, but link with non-Thumb libc.
+	$(Q)$(CC) -o $@ $^ $(LIB) $(LDFLAGS)
 ifndef DEBUG
 	$(Q)$(STRIP) $(STRIPFLAGS_EXTRA) $(PROG)
 endif