[clang] [llvm] Define LLVM_ABI and CLANG_ABI for __EMSCRIPTEN__ builds (PR #131578)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `openmp-s390x-linux` running on `systemz-1` while building `clang,llvm` at step 6 "test-openmp".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/88/builds/9230

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-openmp) failure: 1200 seconds without output running [b'ninja', b'-j 4', b'check-openmp'], attempting to kill
...
PASS: ompd-test :: openmp_examples/example_2.c (442 of 452)
PASS: ompd-test :: openmp_examples/example_4.c (443 of 452)
PASS: ompd-test :: openmp_examples/example_5.c (444 of 452)
PASS: ompd-test :: openmp_examples/example_task.c (445 of 452)
UNSUPPORTED: ompd-test :: openmp_examples/ompd_bt.c (446 of 452)
PASS: ompd-test :: openmp_examples/fibonacci.c (447 of 452)
UNSUPPORTED: ompd-test :: openmp_examples/ompd_parallel.c (448 of 452)
PASS: ompd-test :: openmp_examples/parallel.c (449 of 452)
PASS: ompd-test :: openmp_examples/nested.c (450 of 452)
PASS: ompd-test :: openmp_examples/ompd_icvs.c (451 of 452)
command timed out: 1200 seconds without output running [b'ninja', b'-j 4', b'check-openmp'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1315.180381

```

</details>

https://github.com/llvm/llvm-project/pull/131578