Fix clang 6.0.0 compiler warnings in binutils

Latest clang git has a warning -Wnull-pointer-arithmetic which will
trigger a -Werror failure. Addition and subtraction from a null pointer
is undefined behaviour and could be optimized into anything.

Furthermore, using the difference between two pointers and casting the
result back to a pointer is not portable since the size of ptrdiff_t
does not necessary have to be the same as size of void* (this happens
e.g. on CHERI). Using intptr_t instead fixes this portability issue and
the compiler warning.

Submitted by; Alexander Richardson
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D12928
MFC after: 3 days