Mark the floating-point environment as __gshared

It is not a thread-local variable.
Merge pull request #454 from donc/fenv_gshared

Mark the floating-point environment as __gshared
make Thread.sm_this a plain TLS value

- this became possible, because the TLS emulation
  no longer uses the Thread.getThis() instance

- Windows use impersonate_thread to set sm_this
  when attaching a foreign thread
Merge pull request #456 from dawgfoto/useTLS

make Thread.sm_this a plain TLS value
Revert "Merge pull request #456 from dawgfoto/useTLS"

This reverts commit dee2d7576ca94a21706f64d83ff9b0dfe761a7b1, reversing
changes made to bf0eefe3b2dda5fde25e8070f635e4a8e2deb042.
Merge pull request #457 from AndrejMitrovic/Revert456

Revert "Merge pull request #456 from dawgfoto/useTLS"