improve work queue pipeline to start working on more components in parallel

[andrewrk]

Currently, the only parallelization the main pipeline does is AstGen jobs and C compilation jobs:

zig/src/Compilation.zig

Lines 1913 to 1945 in 8a6de78

	// Here we queue up all the AstGen tasks first, followed by C object compilation.
	// We wait until the AstGen tasks are all completed before proceeding to the
	// (at least for now) single-threaded main work queue. However, C object compilation
	// only needs to be finished by the end of this function.
	var zir_prog_node = main_progress_node.start("AstGen", self.astgen_work_queue.count);
	defer zir_prog_node.end();
	var c_obj_prog_node = main_progress_node.start("Compile C Objects", self.c_source_files.len);
	defer c_obj_prog_node.end();
	self.work_queue_wait_group.reset();
	defer self.work_queue_wait_group.wait();
	{
	self.astgen_wait_group.reset();
	defer self.astgen_wait_group.wait();
	while (self.astgen_work_queue.readItem()) |file| {
	self.astgen_wait_group.start();
	try self.thread_pool.spawn(workerAstGenFile, .{
	self, file, &zir_prog_node, &self.astgen_wait_group,
	});
	}
	while (self.c_object_work_queue.readItem()) |c_object| {
	assert(@ptrToInt(c_object) != 0xaaaa_aaaa_aaaa_aaaa);
	self.work_queue_wait_group.start();
	try self.thread_pool.spawn(workerUpdateCObject, .{
	self, c_object, &c_obj_prog_node, &self.work_queue_wait_group,
	});
	}
	}

If you scroll down and look at the rest of the queue handling, you can see that, among others, the following tasks are done serially:

C/C++ library compilation tasks

• glibc_crt_file
• glibc_shared_objects
• musl_crt_file
• mingw_crt_file
• windows_import_lib
• libunwind
• libcxx
• libcxxabi
• libtsan
• wasi_libc_crt_file
• libssp

zig code compilation tasks:

• compiler_rt_lib
• compiler_rt_obj
• zig_libc
• libfuzzer

At the very least, the C/C++ library compilation tasks should be kicked off in parallel, in the same wait group as C object compilation tasks. The zig compilation tasks can also happen in parallel, but they should wait until after the AstGen tasks are all complete.

[andrewrk]

done in #22541