rtld: populate DT_DEBUG iff DYNAMIC segment is writable

MIPS has/had a read-only DYNAMIC segment, and uses an extra level of
indirection (through MIPS_RLD_MAP) to locate the debugger rendezvous
data.

Some linkers (e.g. LLVM's lld) may produce MIPS binaries with a writable
DYNAMIC segment, which would allow us to eventually drop a special case.

Therefore, instead of hardcoding knowledge that DYNAMIC is not writable
on MIPS just check the permissions on the segment.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4791