devel/heaptrack Try to fix 32-bit build

I don't see that this can ever have built on 32-bit FreeBSD (i386
in particular) because the code uses Sxword, which our elf32.h
doesn't define. The Linux elf.h (which hanbles both sizes) does
define a 64-bit Sxword.

The patch throws in a using (typedef, but C++-style) that mimics
the 64-bit integers used in the Linux headers; this 64-bit value
doesn't match the size of Elf32_Dyn fields in either OS, but
I'll take a warning over non-stop build failures any day.