Fix _mips_rtld_bind() to handle ELF filters.

[jhb]

Summary

Fix _mips_rtld_bind() to handle ELF filters.

MIPS does not use the common _rtld_bind() to handle runtime binding.
Instead, it uses a private _mips_rtld_bind(). Update _mips_rtld_bind()
to include the changes made to _rtld_bind() in r216695 and r218476 to
support upgrading the read-locked rtld_bind_lock to a write lock when
an object with a filter is encountered.

While here, add a 'where' variable to track the location of the fixup
in the GOT to make the code flow more closely match _rtld_bind().

Test Plan

• Ran kib's rtld-filter tests (but not the other tests from the tarball) under mipso32 (mipsn64 binaries failed to link)
• Ran various other programs (linker was installed as the default linker in /) in both mipsn64 and mipso32 qemu instances.
• Ran a gdb7 binary linked against libc++ (but not sure any filters were used as it was built purely with libc++ from scratch so shouldn't reference any libstdc++-specific ABI).

[kib]

Please add a comment to the _rtld_bind() function noting that _mips_rtld_bind() should be kept in sync.