simpler pch support for zig build

[xxxbxxx]

This is a follow-up to #17956 (included here as well), that exposes a simpler interface to use precompiled headers.

It is now possible to simply add the name of the header file to precompile:

exe.addCSourceFiles(.{
            .files = &.{"file.c", ...},
            .flags = ...,
            .precompiled_header = .{ .source_header = .{.path = b.path("all.h")} },
        });

In order to do this,

• I added an extra finalize() function to the build steps,
• created new build steps not explicitly requested by the user,
• as well as altered the compile steps after the user as initialised them.
  I don't know if that fits the general design of the build system or if there's an other way to achieve this.

---

updated example patch for https://github.com/allyourcodebase/cpython:

--- a/build.zig
+++ b/build.zig
@@ -664,7 +664,6 @@ pub fn build(b: *std.Build) void {
         "Modules/getbuildinfo.c",
         "Modules/itertoolsmodule.c",
         "Modules/main.c",
-        "Modules/mathmodule.c",
         "Modules/md5module.c",
         "Modules/sha1module.c",
         "Modules/sha256module.c",
@@ -805,7 +804,19 @@ pub fn build(b: *std.Build) void {
         "-std=c11",
         "-fvisibility=hidden",
         "-DPy_BUILD_CORE",
-    } });
+    }, .precompiled_header = .{ .source_header = .{ .path = b.path("Include/Python.h") } } });
+
+    // files that includes Python.h with NEEDS_PY_IDENTIFIER defined
+    exe.addCSourceFiles(.{ .files = &.{
+        "Modules/mathmodule.c",
+    }, .flags = &.{
+        "-fwrapv",
+        "-std=c11",
+        "-fvisibility=hidden",
+        "-DPy_BUILD_CORE",
+        "-DNEEDS_PY_IDENTIFIER",
+    }, .precompiled_header = .{ .source_header = .{ .path = b.path("Include/Python.h") } } });
+
     exe.addCSourceFiles(.{ .files = &.{
         "Modules/getpath.c",
     }, .flags = &.{