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When cross emerge wrapper is used, using `llvm_prepend_path` without
any options, results in prepending the following path:
```
${ESYSROOT}/usr/lib/llvm/${slot}/bin
```
For example:
```
/usr/aarch64-unknown-linux-musl/usr/lib/llvm/19/bin
```
The problem is that cross emerge wrapper uses a cross clang wrapper as
a compiler, e.g. `aarch64-unknown-linux-musl-clang`. However, the file
with such name can be usually found in two places:
* `/usr/lib/llvm/19/bin/aarch64-unknown-linux-musl-clang`, which is
a clang wrapper that calls the host clang with appropriate
configuration. We want to call that file.
* `/usr/aarch64-unknown-linux-musl/usr/lib/llvm/19/bin/aarch64-unknown-linux-musl-clang`,
which is an actual aarch64 compiler binary. We cannot execute it on
x86_64, or other incompatible host.
However, the path added by `llvm_prepend_path` causes resolution of
`aarch64-unknown-linux-musl-clang` to the second path, therefore causing
errors like:
```
/usr/aarch64-unknown-linux-musl/usr/lib/llvm/19/bin/clang: cannot execute binary file: Exec format error
```
Solve that by:
* Using `LLVM_ROOT` CMake parameter to point the build to LLVM library
artifacts.
* Using `llvm_prepend_path` with `-b` argument if we need clang, which
is the case when either `clang` or `test` USE flags are enabled.
Signed-off-by: Michal Rostecki <vadorovsky@protonmail.com>
Part-of: https://github.com/gentoo/gentoo/pull/39280
Signed-off-by: Michał Górny <mgorny@gentoo.org>
Part-of: https://github.com/gentoo/gentoo/pull/39280
Signed-off-by: Michał Górny <mgorny@gentoo.org>
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