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 fdfe4eca745dce5f20fb65a3c197006b9053999a
a16a330da5