g_part,mkimg: Add additional GPT partition types

Add the hifive-fsbl, hifive-bbl, and xbootldr aliases to mkimg(1).
Add the xbootldr alias to geom(4), and thus gpart(8).

The "hifive" partition types are defined and used by various RISC-V SBCs
for locating firmware.

"xbootldr", or the Extended Boot Loader Partition is defined here:
https://uapi-group.org/specifications/specs/boot_loader_specification/

Reviewed by: emaste, markj, mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D56784