[llvm] [cmake] Fix semicolon expansion when passing LLVM_TABLEGEN_FLAGS (PR #169518)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-ppc64le-linux-multistage` running on `ppc64le-clang-multistage-test` while building `llvm` at step 10 "build stage 2".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/76/builds/13503

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

```
Step 10 (build stage 2) failure: 'ninja' (failure) (timed out)
...
[6293/6594] Linking CXX executable bin/llvm-isel-fuzzer
[6294/6594] Linking C executable bin/clang-fuzzer-dictionary
[6295/6594] Linking CXX executable bin/llvm-gsymutil
[6296/6594] Linking CXX executable bin/llvm-lipo
[6297/6594] Linking CXX executable bin/llvm-c-test
[6298/6594] Linking CXX executable bin/llvm-libtool-darwin
[6299/6594] Linking CXX executable bin/llvm-lto2
[6300/6594] Linking CXX executable bin/opt
[6301/6594] Building CXX object tools/bugpoint-passes/CMakeFiles/BugpointPasses.dir/TestPasses.cpp.o
[6302/6594] Linking CXX shared module lib/BugpointPasses.so
command timed out: 1200 seconds without output running [b'ninja'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1926.040390

```

</details>

https://github.com/llvm/llvm-project/pull/169518