[llvm] cmake: Avoid using if(TARGET x AND TARGET y) (PR #159707)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think the issue is that both variables are empty strings. So `if(TARGET ${llc_target})` will be evaluates as `if(TARGET)`, which I believe is invalid either.

Undefined value is not a target, which works fine 

https://github.com/llvm/llvm-project/pull/159707