[llvm] [PowerPC] Support local-dynamic TLS relocation on AIX (PR #66316)

[Chen Zheng via llvm-commits]

https://github.com/chenzheng1030 commented:

For below case:
```
__thread extern  int x = 42;
__thread extern int y = 42;
int main(void) {
   return x + y;
}
```

For now, we get:
```
.main:
# %bb.0:                                # %entry
        mflr 0
        stwu 1, -32(1)
        stw 0, 40(1)
        lwz 6, L..C0(2)                         # target-flags(ppc-tlsld) @x
        lwz 7, L..C1(2)                         # target-flags(ppc-tlsld) @y
        lwz 3, L..C2(2)                         # target-flags(ppc-tlsld) &"_$TLSML[TC]"
        bla .__tls_get_mod[PR]
        add 3, 3, 6
        lwz 6, 0(3)
        lwz 3, L..C2(2)                         # target-flags(ppc-tlsld) &"_$TLSML[TC]"
        bla .__tls_get_mod[PR]
        add 3, 3, 7
        lwz 3, 0(3)
        add 3, 3, 6
        addi 1, 1, 32
        lwz 0, 8(1)
        mtlr 0
        blr
```

I think we should be able to save one `__tls_get_mod()`?

If this is not easy to optimize now, I'm OK we do this optimization later.

https://github.com/llvm/llvm-project/pull/66316