Define DT_MIPS_RLD_MAP_REL in code, until renamed in sysroot

Gdbserver is using prebuilts/ndk sysroot, where we haven't updated the
tag. There we have old DT_MIPS_RLD_MAP2 tag defined (just a different
name, same value). Until sysroot used to build gdbserver is fixed, we
need to define the tag directly in the source code.  Updating the tag is
the job of: https://android-review.googlesource.com/233345.

Workaround is based on change: I4dd0969c1a6f847fd5b11b1206c7bd6ef1f30137

Verified on: mips, mips64, arm, arm64 emulators.

Change-Id: I552654458d506e219d19c7345d127c6bb2337997