MIPS PCREL support without $gp dependency

MIPS PCREL support without $gp dependency

[YunQiang Su]

I am working on PCREL support for MIPS recently, and the GCC support
has almost finished.

For the new PCREL, I wish that we can drop the $gp dependency.
On the glibc side, in _dl_runtime_resolve , $gp is assumed to have the value of
GOT of our caller.

To be compatible with the current executable and the future executable, we
need to have a way to determine how to get the GOT value.

I have a proposal about it:
1. Use jalr $t6 to call _dl_runtime_resolve, instead of jalr $t6
     and pass the address of GOT with $t9.
2. if  $t6 == ADDR (_dl_runtime_resolve) then
        GOT = $t9
3. else
        GOT = $gp

This way, the old/new executables can work with new _dl_runtime_resolve,
and both old and new executables can work with old _dl_runtime_resolve.

Any better ideas?