infra: fix macOS *san builds

[thibault-martinez]

Building with --config *san on macOS doesn't work properly due to _FORTIFY_SOURCE already being defined, producing a lot of the following warning:

In file included from <built-in>:362:
<command line>:1:9: warning: '_FORTIFY_SOURCE' macro redefined [-Wmacro-redefined]
#define _FORTIFY_SOURCE 1
        ^
<built-in>:349:9: note: previous definition is here
#define _FORTIFY_SOURCE 0
        ^
1 warning generated.

References:
bazelbuild/bazel#6932
google/sanitizers#247

The only reasonable thing is to make sure _FORTIFY_SOURCE is set to 0 when asan/msan/tsan is used.

Test Plan:

CI