This PR fixes two problems with clang/llvm's version detection. clang's
version output looks like this:
```
clang version 11.0.0
Target: x86_64-unknown-linux-gnu
```
This caused clang's version to be misdetected as:
```
clang@11.0.0
Target:
```
This resulted in errors when trying to actually use it as a compiler.
When using `spack external find`, we couldn't determine the compiler
version, resulting in errors like this:
```
==> Warning: "llvm@11.0.0+clang+lld+lldb" has been detected on the system but will not be added to packages.yaml [reason=c compiler not found for llvm@11.0.0+clang+lld+lldb]
```
Changing the regex to only match until the end of the line fixes these
problems.
Fixes: #19473
1b7a5e53a6