build: rework IWYU mapping files

Create mapping to most of the C spec and some POSIX specific functions.
This is more robust than relying files shipped with IWYU.

Makefile

@@ -141,6 +141,7 @@ iwyu: build/.ran-cmake
 	cmake --build build > build/iwyu.log
 	iwyu-fix-includes --only_re="src/nvim" --ignore_re="src/nvim/(auto|map.h|eval/encode.c)" --safe_headers < build/iwyu.log
 	cmake -B build -U ENABLE_IWYU
+	cmake --build build
 
 clean:
 	+test -d build && $(BUILD_TOOL) -C build clean || true