improve MacOS interposes to make mimalloc compile for older MacOS

[Noxybot]

I've got a failure while trying to compile with "-mmacosx-version-min=10.7" since MAC_OS_X_VERSION_MAX_ALLOWED is greater than MAC_OS_X_VERSION_10_15 but still we can't use aligned_alloc since of the lower bound (10.7). I'm not sure about internal details of how __OSX_AVAILABLE works, but my hope is the it would only make this interpose available starting from the OS version specified. Also, it helps me to compile mimalloc with "-mmacosx-version-min=10.7".

[Noxybot]

Hey @daanx - could you check this change once again, please? It makes some interposes more robust (so they are always present when available rather than excluded depending on target MacOS version that mimalloc is compiled for). I've been running both dev2 and dev3 with this change locally for quite some time (and mimalloc seems to not work without it in our environment).

[aeiouaeiouaeiouaeiouaeiouaeiou]

I've got a failure while trying to compile with "-mmacosx-version-min=10.7" since MAC_OS_X_VERSION_MAX_ALLOWED is greater than MAC_OS_X_VERSION_10_15 but still we can't use aligned_alloc since of the lower bound (10.7).

Because MAC_OS_X_VERSION_MAX_ALLOWED forces the current SDK version by default.

[daanx]

I took this PR and manually integrated it. Let me know if this works for you @noxybot. Thanks!