fix: use a bigger real transfering mask_val

this change ensures the type of `mask_val` is big enough to satisfy the
`transfer`; otherwise, we risk transfering a 4-byte value to 2-element
arrays of 4-byte elements (i.e., `i4tmp(2)` and `r4tmp(2)`)

I encountered this issue compiling the project with Flang; see
https://godbolt.org/z/91EfY154n

mpp/include/mpp_chksum_int.fh

@@ -52,6 +52,7 @@ function MPP_CHKSUM_INT_RMASK_( var, pelist, mask_val )
  MPP_TYPE_, intent(in) :: var MPP_RANK_
  integer, optional :: pelist(:)
  real, intent(in) :: mask_val
+ real(KIND=r8_kind) :: tmp_mask_val
  integer(KIND(var))::imask_val
  integer(KIND=i4_kind)::i4tmp(2)=0
  real(KIND=r4_kind)::r4tmp(2)=0
@@ -74,9 +75,10 @@ function MPP_CHKSUM_INT_RMASK_( var, pelist, mask_val )
 ! Secondary Logic:
 !! We've done something dangerous
  else
- i8tmp = TRANSFER(mask_val , i8tmp )
- i4tmp = TRANSFER(mask_val , i4tmp )
- r4tmp = TRANSFER(mask_val , r4tmp )
+ tmp_mask_val = mask_val
+ i8tmp = TRANSFER(tmp_mask_val , i8tmp )
+ i4tmp = TRANSFER(tmp_mask_val , i4tmp )
+ r4tmp = TRANSFER(tmp_mask_val , r4tmp )
  if ( i8tmp == MPP_FILL_INT ) then
  ! we've packed an MPP_FILL_
  imask_val = MPP_FILL_INT