support win7 target

[zhangli-pear]

Add support for *-win7-windows-msvc.

[Fenex]

I've tried to compile a modern rustc as host compiler for running on win7 and facing with the same error.

However I think its will be better to skip checks for target_components()[1] (known as target_vendor) at all due vendors can be various, but this all is msvc actually. Before we have "pc" only, right now we want to add "win7". Perhaps, in the future we need to add something else. At least for now there is a "rust9x" target vendor to compile for Windows Vista and older (down to 9x/ME).

tldr, my proposal to use this function:

fn is_windows_msvc() -> bool {
    // Some targets are only two components long, so check in steps.
    target_components().len() > 2
        && target_components()[2] == "windows"
        && target_components()[3] == "msvc"
}

or maybe even (it's required to deeper research for correctness):

fn is_windows_msvc() -> bool {
    // Some targets are only two components long, so check for length before access to ABI target component.
    target_components().len() == 4 && target_components()[3] == "msvc"
}