Platform-dependent `subt`/`tsub` bug when compiling with `-flto`

[mattsignorelli]

Nasty bug, only seen when specifically using the Mac and Windows platforms in Docker within BinaryBuilder (cannot reproduce when compiling on my Mac), but the binaries are made within the Docker so this is a real problem

[mattsignorelli]

Possibly other incorrectness, this is with DESC_USE_TMP=1, haven't tried DESC_USE_TMP=0

[mattsignorelli]

confirm bug goes away at least in Mac when excluding flto but including all other flags

[mattsignorelli]

flto is safe (all tests pass) when DESC_USE_TMP=0. Therefore the bug is related to how link-time optimization handles getting/using/releasing permanent temporaries in the descriptor

For now I will compile without lto, but the long term solution is DESC_USE_TMP=0 and use a thread-safe allocator as Laurent has implemented

[mattsignorelli]

Even with lowest optimization levels and no vectorization, bug appears with LTO