arm64/clang(?) Pressure Floor

Cannot link on arm64 (modern MacOS systems) due to pressure floor issues. Currently using clang as the compiler.

Configuring with ./configure --disable-hand-vec --disable-mpi.

Error:

Undefined symbols for architecture arm64:
  "_pressure_floor_props", referenced from:
      _main in swift-main.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

I've tried this with pressure floors other than None, and I've tried sticking random variables into the pressure_floor_props and initialising them. Neither seems to work.

A problem here is that the commits are often broken for other reasons, like they have broken function calls etc. in them, meaning it took a lot of work to bisect to: a097696ae14a8d4c0a1a4e82bfb35f7b42fec515.