Mark the floating-point environment as __gshared

It is not a thread-local variable.
In the following line, it takes the address of the variable and puts it into an enum. That doesn't work; attempting to compile the module generates a "non-constant expression" in the glue layer. Interestingly, the bug was already fixed on FreeBSD. It doesn't apply on Linux.

This is a blocker for my CTFE refactoring, beginning with this pull:
https://github.com/D-Programming-Language/dmd/pull/1763