rustdoc: change doctests locating rustc binary

[Mark-Simulacrum]

We previously used the "naive" approach of replacing the current_exe()'s file name with rustc, but now load from the sysroot by default ($sysroot/bin/rustc). The functionality of locating the sysroot overlaps/is the same as the functionality used by codegen backend loading; this ensures that any failure cases we've introduced are not exceeding those, and that improvements to finding the sysroot for loading codegen backends likewise enhance rustdoc.

The second commit adds an unstable --test-builder flag to rustdoc, and is largely separate (I can split into separate PR, but it's a simple and related change). This is largely intended for "advanced" users at this point (I'm not sure if we'll ever stabilize it); it permits use of a different rustc binary for rustdoc compilation of doctests than the rustdoc binary used when loading. Note, that this may not be what you want as the parsers and such differ (and rustdoc uses its own libsyntax, etc.). However, I've been told that running doctests in miri may be assisted by this change, so I've implemented it; I'll file a tracking issue for it if there's interest in it (and we land this PR).