rtld-elf: Check the return value of obj_enforce_relro()

[arichardson]

Summary

The mprotect() call was failing on CheriBSD when changing rtld's relro
page permissions due to missing CHERI capability permissions on the
mprotect() argument but did not report an error since the return value
was being ignored. It should never fail on any supported FreeBSD
architecture, but checking the return value seems like a good
sanity check to me.

[arichardson]

libexec/rtld-elf/rtld.c

2291–2292

Indentation is a bit odd in this entire function, I just used the existing style.

[kib]

libexec/rtld-elf/rtld.c

2291–2292

Other users of obj_enforce_relro() check for -1 to detect error.

[arichardson]

libexec/rtld-elf/rtld.c

2291–2292

Ah yes, best to be consistent. @kib okay to commit with != 0 changed to == -1?

[kib]

libexec/rtld-elf/rtld.c

2291–2292

Sure, ok.