Bug 1083020 - Add a (dummy) stats method to all LibHandles. r=nfroyd

While here, avoid doing anything if debug logging is disabled.
Bug 1078837 part 1 - Move CustomElf::mappable to BaseElf. r=nfroyd
Bug 1078837 part 2 - Replace IsSystemElf/reinterpret_cast dance with better API. r=nfroyd
Bug 1081034 part 1 - Move GetSymbolPtr, Contains and FindExidx from CustomElf to BaseElf. r=nfroyd
Bug 1081034 part 2 - Move initialization of self_elf to its own separate class. r=nfroyd

The new class is kind of like SystemElf, but using our linker's own symbol
resolution. This also adds some initialization from ELF program headers that
weren't done previously for self_elf, as well as registration as for CustomElf
instances.
Bug 1081034 part 3 - Resolve libc symbols with our linker. r=nfroyd

This allows to resolve weak symbols from some Android device's libc that
dlsym() won't. This is effectively an alternative fix to bug 791419, without
requiring wrapping symbols.
bug 1081034 part 4 - Remove wrapping for weak libc symbols. r=nfroyd

This effectively backs out bug 791419, a part of bug 850332, and bug 1001703.