[Flang][OpenMP] Use internal linkage for OpenMP code-gen'ed helper functions (#117911)

When compiling WORKSHARE construct in different compilation units, a
linker error happened, when two equal WORKSHARE constructs with a copy
operation have been compiled:

```
/usr/bin/ld: module2.o: in function `_workshare_copy_f64':
FIRModule:(.text+0x0): multiple definition of `_workshare_copy_f64'; module1.o:FIRModule:(.text+0x0): first defined here
```

Reason is that the generate copy function has the wrong linkage:

```
0000000000000000 T _workshare_copy_f64
```

while it should be

```
0000000000000000 t _workshare_copy_f64
```

0 files changed, 0 insertions, 0 deletions