cmELF: Fix check for TagMipsRldMapRel

DT_MIPS_RLD_MAP_REL is a machine-speicific dynamic tag, so other
architectures could re-use the value of 0x70000035 to mean something
else.  Before using DT_MIPS_RLD_MAP_REL, we have to check that the ELF
file is actually has a e_machine of EM_MIPS.