src: unify Linux and FreeBSD large pages implem

dl_iterate_phdr(3) is also available for FreeBSD. This change adds the same trimming code for the start and end of the .text section as on Linux, making it work on FreeBSD, and removing the need for the additional FreeBSD-specific check.

Manually tested on

• https://www.osboxes.org/freebsd/#freebsd-12-1-vbox
• https://www.osboxes.org/freebsd/#freebsd-11-vbox
• test-digitalocean-freebsd11-x64-2

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines