CryptoPkg: Refine type cast for pointer subtraction
    
    For pointer subtraction, the result is of type "ptrdiff_t". According to
    the C11 standard (Committee Draft - April 12, 2011):
    
    "When two pointers are subtracted, both shall point to elements of the
    same array object, or one past the last element of the array object; the
    result is the difference of the subscripts of the two array elements. The
    size of the result is implementation-defined, and its type (a signed
    integer type) is ptrdiff_t defined in the <stddef.h> header. If the result
    is not representable in an object of that type, the behavior is
    undefined."
    
    In our codes, there are cases that the pointer subtraction is not
    performed by pointers to elements of the same array object. This might
    lead to potential issues, since the behavior is undefined according to C11
    standard.
    
    Also, since the size of type "ptrdiff_t" is implementation-defined. Some
    static code checkers may warn that the pointer subtraction might underflow
    first and then being cast to a bigger size. For example:
    
    UINT8  *Ptr1, *Ptr2;
    UINTN  PtrDiff;
    ...
    PtrDiff = (UINTN) (Ptr1 - Ptr2);
    
    The commit will refine the pointer subtraction expressions by casting each
    pointer to UINTN first and then perform the subtraction:
    
    PtrDiff = (UINTN) Ptr1 - (UINTN) Ptr2;
    
    Contributed-under: TianoCore Contribution Agreement 1.0
    Signed-off-by: Hao Wu <hao.a.wu@intel.com>
    Acked-by: Laszlo Ersek <lersek@redhat.com>
    Reviewed-by: Qin Long <qin.long@intel.com>