Prefer _WIDTH macros to sizeof in pdumper.c

Prefer _WIDTH macros to sizeof in pdumper.c

This is a bit clearer, and should work better on hypothetical
platforms where integers have holes in their representation.
* src/pdumper.c: Since the code no longer uses CHAR_BIT,
don’t worry about whether it equals 8.
(DUMP_OFF_WIDTH): New macro.
(EMACS_RELOC_LENGTH_BITS, DUMP_RELOC_OFFSET_BITS): Use it.
(DUMP_BITSET_WORD_WIDTH): New macro.
(dump_bitsets_init, dump_bitset__bit_slot)
(dump_bitset_bit_set_p, dump_bitset__set_bit_value): Use it.