Makefile: Enable address sanitizer with DEBUG_ASAN=1

The Address Sanitizer (built in GCC and LLVM) is an extremely powerful
tool, moreso than e.g. Valgrind for detecting memory issues.

It is not available on every architecture/OS combination (see
https://github.com/google/sanitizers/wiki/AddressSanitizer), so it can
only be enabled by compiling with DEBUG_ASAN=1.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>