This makes it easier to debug testcases:
- the entity numbers in a .cton file match the entity numbers used
within Cretonne.
- serializing and deserializing doesn't cause indices to change.
One disadvantage is that if a .cton file uses sparse entity numbers,
deserializing to the in-memory form doesn't compact it. However, the
text format is not intended to be performance-critical, so this isn't
expected to be a big burden.
Add an addend field to reloc_external, and use it to move the
responsibility for accounting for the difference between the end of an
instruction (where the PC is considered to be in PC-relative on intel)
and the beginning of the immediate field into the encoding code.
Specifically, this makes IntelGOTPCRel4 directly correspond to
R_X86_64_GOTPCREL, instead of also carrying an implicit `- 4`.
