Make dwarf_vma uint64_t

Make dwarf_vma uint64_t

This replaces dwarf_vma, dwarf_size_type and dwarf_signed_vma with
uint64_t and int64_t everywhere.  The patch also gets rid of
DWARF_VMA_FMT since we can't use that with uint64_t, and all of the
configure support for deciding the flavour of HOST_WIDEST_INT.
dwarf_vmatoa also disappears, replacing most uses with one of
PRIx64, PRId64 or PRIu64.  Printing of size_t and ptrdiff_t values
now use %z and %t rather than by casting to unsigned long.  Also,
most warning messages that used 0x%lx or similar now use %#lx and a
few that didn't print the 0x hex prefix now also use %#.  The patch
doesn't change normal readelf output, except in odd cases where values
previously might have been truncated.