[llvm] Allow specifying libcxx builder image. (PR #110303)

[Louis Dionne via llvm-commits]

ldionne wrote:

Thanks a lot for chiming in! Side question: I thought ASLR was the problem, so shouldn't the solution be to **turn off** ASLR, i.e. use `settings set target.disable-aslr true`? The Discord thread seems to have slightly conflicting information.

https://github.com/llvm/llvm-project/pull/110303