humility: Begin target addr agnosticism

[jamesmunns]

This PR is the Minimum Viable Change necessary to make things Just Work with oxidecomputer/hubris#2703.

This does not get us all the way to target-bitwidth agnosticism, but makes humility dumps continue to work both with existing, pre-Addr changes, as well as with post-2703 images.

It is IMO, somewhat icky to half-land these features, but the "u32 is a (target) address" is pervasive in humility, humpty, and even in hubris beyond what 2703 touches, so I think pragmatically we will want to carefully land these changes incrementally.

With a 2703-containing sidecar image, and on ~current humility master:

jamesmunns@lurch ~ $ pfexec ./humility0923 -a /staff/jamesmunns/sidecar-b-abi-less.zip -t sidecar-sp dump
humility: WARNING: archive on command-line overriding archive in environment file
humility: attached to 0483:374e:0028001E4741500720383733 via ST-Link V3
humility: core halted
humility: dumping to "hubris.core.4"
humility: core resumed
humility dump failed: expected base in struct RegionDesc (GOFF 0x00013ae7) to be 4 byte type, found to be GOFF 0x00022202

With ~current master sidecar image, and on this branch:

jamesmunns@lurch ~ $ pfexec ./humility-plz-0929 -t sidecar-sp dump
humility: attached to 0483:374e:0028001E4741500720383733 via ST-Link V3
humility: core halted
humility: dumping to "hubris.core.8"
humility: dumped 1.11 MiB in 5 seconds
humility: core resumed

With a 2703-containing sidecar image, and on this branch:

jamesmunns@lurch ~ $ pfexec ./humility-plz-0929 -a /staff/jamesmunns/sidecar-b-abi-less.zip -t sidecar-sp dump
humility: WARNING: archive on command-line overriding archive in environment file
humility: attached to 0483:374e:0028001E4741500720383733 via ST-Link V3
humility: core halted
humility: dumping to "hubris.core.7"
humility: dumped 1.07 MiB in 5 seconds
humility: core resumed