nrf,stm32: Don't enable debug info by default if LTO is on.

It seems sometimes gcc with LTO will generate otherwise valid assembly
listings that cause 'as' to error out when generating DWARF debug info; see
https://sourceware.org/bugzilla/show_bug.cgi?id=29494

Therefore, don't enable -g by default if LTO is on.

Enabling LTO=1 DEBUG=1 is still possible but may result in random errors
at link time due to 'as' (the error in this case is "Error: unaligned
opcodes detected in executable segment", and the only other easy workaround
is CFLAGS+=-fno-jump-tables which may increase code size significantly).

Follows on from fdfe4eca74

ports/nrf/Makefile

@@ -138,12 +138,14 @@ LDFLAGS += -Wl,'--defsym=_fs_size=$(FS_SIZE)'
 endif
 
 #Debugging/Optimization
-CFLAGS += -g  # always include debug info in the ELF
 ifeq ($(DEBUG), 1)
 #ASMFLAGS += -g -gtabs+
-CFLAGS += -O0
+CFLAGS += -g -O0
 LDFLAGS += -O0
 else
+ifneq ($(LTO), 1)
+CFLAGS += -g  # always include debug info in the ELF, unless LTO is on
+endif
 CFLAGS += -Os -DNDEBUG
 LDFLAGS += -Os
 endif