[flang] Modifications to ieee floating point environment procedures (#121949)

Intrinsic module procedures ieee_get_modes, ieee_set_modes,
ieee_get_status, and ieee_set_status store and retrieve opaque data
values whose size varies by machine and OS environment. These data
values are usually, but not always small. Their sizes are not directly
known in a cross compilation environment. Address this issue by
implementing two mechanisms for processing these data values.
Environments that use typical small data sizes can access storage
defined at compile time. When this is not valid, data storage of any
size can be allocated at runtime.

flang/module/__fortran_ieee_exceptions.f90

@@ -36,13 +36,15 @@ module __fortran_ieee_exceptions
     ieee_all(*) = [ ieee_usual, ieee_underflow, ieee_inexact ]
 
   type, public :: ieee_modes_type ! Fortran 2018, 17.7
-    private ! opaque fenv.h femode_t data
+    private ! opaque fenv.h femode_t data; code will access only one component
     integer(kind=4) :: __data(_FORTRAN_RUNTIME_IEEE_FEMODE_T_EXTENT)
+    integer(kind=1), allocatable :: __allocatable_data(:)
   end type ieee_modes_type
 
   type, public :: ieee_status_type ! Fortran 2018, 17.7
-    private ! opaque fenv.h fenv_t data
+    private ! opaque fenv.h fenv_t data; code will access only one component
     integer(kind=4) :: __data(_FORTRAN_RUNTIME_IEEE_FENV_T_EXTENT)
+    integer(kind=1), allocatable :: __allocatable_data(:)
   end type ieee_status_type
 
 ! Define specifics with 1 LOGICAL or REAL argument for generic G.