[wasm] Exclude builtin headers from WASILibc system module

stable/20230725 now rejects including builtin headers in modulemap by
default to avoid module cycles on non-Apple platforms. Builtin headers
are now part of their own `_Builtin_XXX` modules.

See:
- swiftlang/llvm-project#7618
- https://reviews.llvm.org/D159483
- https://reviews.llvm.org/D159064

Author: kateinoigakukun

stdlib/public/Platform/wasi-libc.modulemap

@@ -19,20 +19,12 @@ module SwiftWASILibc [system] {
   header "ctype.h"
   header "errno.h"
   header "fenv.h"
-  header "float.h"
   header "inttypes.h"
-  header "iso646.h"
-  header "limits.h"
   header "locale.h"
   header "math.h"
-  header "stdarg.h"
-  header "stdbool.h"
-  header "stddef.h"
-  header "stdint.h"
   header "stdio.h"
   header "stdlib.h"
   header "string.h"
-  header "tgmath.h"
   header "time.h"
 
   // POSIX