[LLD][PowerPC] Fix bug in PC-Relative initial exec

There is a bug when initial exec is relaxed to local exec.
In the following situation:

InitExec.c
```
extern __thread unsigned TGlobal;
unsigned getConst(unsigned*);
unsigned addVal(unsigned, unsigned*);

unsigned GetAddrT() {
  return addVal(getConst(&TGlobal), &TGlobal);
}
```

Def.c
```
__thread unsigned TGlobal;

unsigned getConst(unsigned* A) {
  return *A + 3;
}

unsigned addVal(unsigned A, unsigned* B) {
  return A + *B;
}
```

The problem is in InitExec.c but Def.c is required if you want to link the example and see the problem.
To compile everything:
```
clang -O3 -mcpu=pwr10 -c InitExec.c
clang -O3 -mcpu=pwr10 -c Def.c
ld.lld InitExec.o Def.o -o IeToLe
```

If you objdump the problem object file:
```
$ llvm-objdump -dr --mcpu=pwr10 InitExec.o
```
you will get the following assembly:
```
0000000000000000 <GetAddrT>:
       0: a6 02 08 7c  	mflr 0
       4: f0 ff c1 fb  	std 30, -16(1)
       8: 10 00 01 f8  	std 0, 16(1)
       c: d1 ff 21 f8  	...