[llvm] [cmake] LLVM_BUILTINS_TARGET='default' should bootstrap builtins without compiler-rt (PR #171536)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-flang-rhel-clang` running on `ppc64le-flang-rhel-test` while building `llvm` at step 6 "test-build-unified-tree-check-flang".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/157/builds/42895

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

```
Step 6 (test-build-unified-tree-check-flang) failure: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
...
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/83/150 (4038 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/76/150 (4039 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/97/150 (4040 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/93/150 (4041 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/94/150 (4042 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/95/150 (4043 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/57/150 (4044 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/99/150 (4045 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/52/150 (4046 of 4048)
PASS: flang-Unit :: Optimizer/./FlangOptimizerTests/47/150 (4047 of 4048)
command timed out: 1200 seconds without output running [b'ninja', b'check-flang'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=2317.942957

```

</details>

https://github.com/llvm/llvm-project/pull/171536