btxldr: process all PT_LOAD segments, not just the first two

With default settings GNU ld generates two PT_LOADs for loader.sym while
LLD generates three, because it creates a rodata segment. Previously
btxldr terminated phdr processing after two PT_LOADs. Remove the early
termination to process all PT_LOADs.

Reviewed by: kib, tsoome
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D8929