spirv: drive capability and extension emission from target features

- Emitting `OpCapability` and `OpExtension` from assembly is no longer allowed (tbh it never was because we silently ignored them).
  They now `fail` with a message pointing users at `-mcpu`.

- `tools/gen_spirv_spec.zig` now also emits `Extension` and the `Capability.dependencies()`.
  Also To keep the feature set a reasonable size, the generator filters to an allowlist of `KHR` and `EXT` entries.
  We can decide which vendors to allow later if a use-case is found.

- `tools/update_cpu_features.zig` now uses the generated `spec.zig` and emits a  feature for every
  capability and extension, with the capability's `.deps` populated from `Capability.dependencies()`.

- `Module.finalize`, `Module.entryPoints`, `Module.addCapability`, `Module.addExtension`
  and `sections.capabilities`/`sections.extensions` are deleted.

- Linker now verifies that an `.spv` object declares the `Linkage` capability

build.zig

@@ -70,6 +70,23 @@ pub fn build(b: *std.Build) !void {
         b.getInstallStep().dependOn(&install_std_docs.step);
     }
 
+    const update_cpu_features = b.addExecutable(.{
+        .name = "update-cpu-features",
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("tools/update_cpu_features.zig"),
+            .target = b.graph.host,
+            .imports = &.{.{
+                .name = "spirv_spec",
+                .module = b.createModule(.{
+                    .root_source_file = b.path("src/codegen/spirv/spec.zig"),
+                    .target = b.graph.host,
+                }),
+            }},
+        }),
+    });
+    const run_update_cpu_features = b.addRunArtifact(update_cpu_features);
+    run_update_cpu_features.addPassthruArgs();
+
     if (flat) {
         b.installFile("LICENSE", "LICENSE");
         b.installFile("README.md", "README.md");
@@ -85,6 +102,9 @@ pub fn build(b: *std.Build) !void {
     docs_step.dependOn(langref_step);
     docs_step.dependOn(std_docs_step);
 
+    const update_cpu_features_step = b.step("update-cpu-features", "Update CPU Features");
+    update_cpu_features_step.dependOn(&run_update_cpu_features.step);
+
     const no_matrix = b.option(bool, "no-matrix", "Limit test matrix to exactly one target configuration") orelse false;
     const fuzz_only = b.option(bool, "fuzz-only", "Limit test matrix to one target suitable for fuzzing") orelse false;
     const skip_debug = b.option(bool, "skip-debug", "Main test suite skips debug builds") orelse false;