[llvm] [LoongArch] Support emulated TLS (PR #92483)

[via llvm-commits]

wangleiat wrote:

Hi @MaskRay  
  Previous discussion: https://reviews.llvm.org/D141183. Despite the numerous drawbacks of emulated TLS, there is still some demand for it. If you still insist that the LoongArch backend should not support emulated TLS, then we should at least provide users with clear warnings either in the frontend or during the downgrading process.
Perhaps like this:

```
if (DAG.getTarget().useEmulatedTLS())
  report_fatal_error("The emulated TLS is prohibited");
```
Otherwise, this issue will be reported as 'some symbols undefined' errors during the linking phase, which can be very confusing.

https://github.com/llvm/llvm-project/pull/92483