[flang] Adjust _FORTRAN_RUNTIME_IEEE_FENV_T_EXTENT for Solaris (#74590)

Even after 13e2200f (Solaris lacks
`femode_t`, too), the Solaris `flang` build is still broken:
```
/vol/llvm/src/llvm-project/local/flang/runtime/exceptions.cpp:87:5: error: static assertion failed due to requirement 'sizeof(fenv_t) <= sizeof(int) * 8': increase ieee_status_type size
   87 |     sizeof(fenv_t) <= sizeof(int) * _FORTRAN_RUNTIME_IEEE_FENV_T_EXTENT,      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/vol/llvm/src/llvm-project/local/flang/runtime/exceptions.cpp:87:20: note: expression evaluates to '200 <= 32'
   87 |     sizeof(fenv_t) <= sizeof(int) * _FORTRAN_RUNTIME_IEEE_FENV_T_EXTENT,      |     ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```
This patch fixes this by removing the assertion.

Tested on `amd64-pc-solaris2.11` and `sparcv9-sun-solaris2.11`.