Handle the DT_MIPS_RLD_MAP_REL dynamic tag in RTLD

This dynamic tag contains the location of the .rld_map section relative to
the location of the dynamic tag. For PIE MIPS binaries DT_MIPS_RLD_MAP can
not be used since it contains an absolute address. Without this change
GDB can not find the function program counters in other libraries and once
I apply this change I can successfully run info sharedlibraries again.

Reviewed By: kib
Differential Revision: https://reviews.freebsd.org/D17867