fixes for compile errors on macOS 10.13 with XCode 10.1

[michaeljclark]

some changes that were required to compile mimalloc on macOS 10.13:

• fix compiler error due to const argument to mi_atomic_load_relaxed
  • seems the address to atomic_load_relaxed should be const so perhaps this is a bug in the XCode 10.1 <stdatomic.h> header, nevertheless, a change is needed to get mimalloc to compile with this version of the XCode C library headers. an alternative fix may be to make the segment pointer non-const.
  • reference: https://en.cppreference.com/w/c/atomic/atomic_load
• fix compiler error claimed_address is not present in malloc_zone_t until macOS 10.14
  • searched through the libmalloc versions to verify this field first appears in macOS 10.14.
• fix compiler error aligned_alloc is not present in C on any version of macOS
  • the function appears in XcodeDefault.xctoolchain/usr/include/c++/v1/stdlib.h but not in any of the SDK /usr/include/stdlib.h headers, so one presumes it is only available in C++ on macOS.

$ clang --version
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

[ghost]

All CLA requirements met.

[daanx]

Thanks @michaeljclark ! I missed this earlier and already fixed the const warning issue. Are you sure about aligned_alloc? It seems to work fine on my M1 with Big Sur. Perhaps we should wrap it in an #ifdef __cplusplus ?

Since this leaves just the claimed_address, I will fix this directly in dev if you are ok.

Thanks again!