Changes

Sanitizer(UBSAN): really enable UBSAN

The clang-14 docs are not clear about this, but the latest docs say:
> -fsanitize-trap= and -fsanitize-recover= are a no-op in the absence of a -fsanitize= option. There is no unused command line option warning.

Avoid this trap and specify `-fsanitize=$ubsan` too.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
 (commit: 1cb3666)

Sanitizers(UBSAN): fix linker warning

Apparently >-O0 needs to be supplied when linking too:
```
clang: warning: the object size sanitizer has no effect at -O0, but is explicitly enabled: -fsanitize=bool,builtin,bounds,enum,nonnull-attribute,nullability,object-size,pointer-overflow,returns-nonnull-attribute,shift-exponent,unreachable [-Winvalid-command-line-argument]
```

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
 (commit: 0a88f1e)

Sanitizers: move flag definitions

Move compiler flag definitions to shell variables.
This will enable testing them.

No functional change yet.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
 (commit: c3b3464)

Sanitizers: add a test that ASAN/UBSAN work

Compile 2 small test programs, and check expected output.
The test programs are deleted on success.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
 (commit: 52d93a5)

Sanitizer(UBSAN): switch to printing instead of trap

Still stop on first error, but instead of dumping core print a message,
and stacktrace, like ASAN.

This may be more convenient when used in a CI than a coredump.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
 (commit: 9f7aad8)

Sanitizers: add -g flag

For better stacktraces.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
 (commit: 9addfad)

Sanitizers: future proof by using -Og instead of -o1

This is currently equivalent to -O1 on clang, but may improve debuggability in the future compared to -O1.

GCC already disables certain optimizations at -Og, and recommends it when using the sanitizers:
```
To get more accurate stack traces, it is possible to use options such as -O0, -O1, or -Og (which, for instance, prevent  most  function  inlining)
```

(there are also other flag recommendations, but I think they are only relevant if you want to use -O2 or higher,
 they wouldn't be active at -O1/-Og)

Also don't use -O0. Even for pure debugging, the manual recommends -Og:
```
It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0
```

Signed-off-by: Edwin Török <edwin.torok@cloud.com>
 (commit: 7c08154)