Specify GCC prefix in LLVM-based compilers (#33146)

* spack.compiler.Compiler: introduce prefix property

We currently don't really have something that gives the GCC install
path, which is used by many LLVM-based compilers (llvm, llvm-amdgpu,
nvhpc, ...) to fix the GCC toolchain once and for all.

This `prefix` property is dynamic in the sense that it queries the
compiler itself. This is necessary because it's not easy to deduce the
install path from the `cc` property (might be a symlink, might be a
filename like `gcc` which works by having the compiler load a module
that sets the PATH variable, might be a generic compiler wrapper based
on environment variables like on cray...).

With this property introduced, we can clean up some recipes that have
the logic repeated for GCC.

* intel-oneapi-compilers: set --gcc-sysroot to %gcc prefix

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