Bootstrap build failing on Windows (MSVC)

[BlaCoiso]

Commit 418b1fa broke builds on Windows as llvm-config has a .exe extension but

rust/src/bootstrap/native.rs

Lines 163 to 164 in 418b1fa

	let llvm_config = llvm_root.join("bin/llvm-config");
	t!(filetime::set_file_times(&llvm_config, now, now));

does not take that into account, resulting in a file not found error and bootstrap panicking.

Patching line 163 with

let llvm_config = llvm_root.join("bin").join(exe("llvm-config", builder.config.build));

fixes the build.

[jyn514]

@BlaCoiso do you want to make a PR with that patch? :)