add Darwin support for detecting maxrss of child process

[andrewrk]

Extracted from #14647.

zig/lib/std/child_process.zig

Lines 75 to 110 in b4d58e9

	/// Set to true to obtain rusage information for the child process.
	/// Depending on the target platform and implementation status, the
	/// requested statistics may or may not be available. If they are
	/// available, then the `resource_usage_statistics` field will be populated
	/// after calling `wait`.
	/// On Linux, this obtains rusage statistics from wait4().
	request_resource_usage_statistics: bool = false,
	/// This is available after calling wait if
	/// `request_resource_usage_statistics` was set to `true` before calling
	/// `spawn`.
	resource_usage_statistics: ResourceUsageStatistics = .{},
	pub const ResourceUsageStatistics = struct {
	rusage: @TypeOf(rusage_init) = rusage_init,
	/// Returns the peak resident set size of the child process, in bytes,
	/// if available.
	pub inline fn getMaxRss(rus: ResourceUsageStatistics) ?usize {
	switch (builtin.os.tag) {
	.linux => {
	if (rus.rusage) |ru| {
	return @intCast(usize, ru.maxrss) * 1024;
	} else {
	return null;
	}
	},
	else => return null,
	}
	}
	const rusage_init = switch (builtin.os.tag) {
	.linux => @as(?std.os.rusage, null),
	else => {},
	};
	};

zig/lib/std/child_process.zig

Lines 374 to 383 in b4d58e9

	const res: os.WaitPidResult = res: {
	if (builtin.os.tag == .linux and self.request_resource_usage_statistics) {
	var ru: std.os.rusage = undefined;
	const res = os.wait4(self.id, 0, &ru);
	self.resource_usage_statistics.rusage = ru;
	break :res res;
	}
	break :res os.waitpid(self.id, 0);
	};

Related: #14956