Stage0 local rust compatibility checking

[MagaTailor]

With the addition of more bootstrap options it would be useful to check local rustc suitability already at configure time.

In the past, an outdated stage0 snapshot (via --enable-local-rust) would barf right away during libcore compilation but nowadays it seems to be more complicated. I've just received reams of errors from as far as libsyntax that, I suspect, must come from incompatible rustc used. A few examples, just to make sure it's no longer #usable:

src/libsyntax/ext/quote.rs:597:64: 597:91 error: failed to resolve. Could not find `Ident` in `ast` [E0433]
src/libsyntax/ext/quote.rs:597                 Some(name) => cx.expr_some(sp, mk_name(cx, sp, ast::Ident::with_empty_ctxt(name)))

src/libsyntax/ext/quote.rs:776:17: 776:41 error: failed to resolve. Could not find `KleeneOp` in `ast` [E0433]
src/libsyntax/ext/quote.rs:776                 ast::KleeneOp::OneOrMore => "OneOrMore",
                                               ^~~~~~~~~~~~~~~~~~~~~~~~

src/libsyntax/ext/quote.rs:917:46: 917:55 error: type name `ast::Expr` is undefined or not in scope [E0412]
src/libsyntax/ext/quote.rs:917                      tts: &[TokenTree]) -> P<ast::Expr> {
                                                                            ^~~~~~~~~

src/libsyntax/ext/quote.rs:64:5: 71:6 error: conflicting implementations of trait `ext::quote::rt::ToTokens`: [E0119]

src/libsyntax/ext/tt/macro_rules.rs:141:1: 158:2 error: conflicting implementations of trait `ext::base::TTMacroExpander` for type `ext::tt::macro_rules::MacroRulesMacroExpander`: [E0119]

and so on. Preferably, an error message could be printed immediately on rustc version check.

Additonally, if a full llvm build is done first, a scenario like that won't lead to lots of wasted time, in case the rustc used (even the official download) is incompatible or crashing (e.g. SIGILL, SIGSEGV):
#33928
#33483

[MagaTailor]

#31322 (comment)