Centralize environ setup.

[jsirois]

Move processing of std.os.environ centrally to std.os and provide a
hook for discovering the raw environ block for libraries that don't
link libc.

Fixes #23372
Affects #4524

[jsirois]

With this, my #23372 use case sees the following compile error:

install
└─ install pexcz
   └─ zig build-lib pexcz Debug native 1 errors
/home/jsirois/dev/ziglang/zig/lib/std/os.zig:82:21: error: missing std.options.environ_setupFn: there is no way to collect environment variables.
                    @compileError(
                    ^~~~~~~~~~~~~

And fixes it in its root file with:

pub const std_options: std.Options = .{
    .environ_setupFn = if (builtin.link_libc) null else posixEnviron,
};

var environ: ?[*:null]?[*:0]u8 = null;

fn posixEnviron() [*:null]?[*:0]u8 {
    return environ.?;
}

// N.B.: This is the main entry point called by Python code via ctypes.
fn bootPosix(
    python: [*:0]const u8,
    pex: [*:0]const u8,
    original_argv: [*:null]?[*:0]const u8,
    envp: [*:null]?[*:0]u8,
) callconv(.c) c_int {
    environ = envp;
    // ...
}