[wasm][stdlib] Enable wasi-libc emulations to build stdlib

But I'm not sure all of them are actually needed.

Author: kateinoigakukun

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -323,7 +323,7 @@ function(_add_target_variant_c_compile_flags)
   endif()
 
   if("${CFLAGS_SDK}" STREQUAL "WASI")
-    list(APPEND result "-D_WASI_EMULATED_MMAN" "-D_WASI_EMULATED_PROCESS_CLOCKS")
+    list(APPEND result "-D_WASI_EMULATED_MMAN" "-D_WASI_EMULATED_SIGNAL" "-D_WASI_EMULATED_PROCESS_CLOCKS")
   endif()
 
   if(NOT SWIFT_STDLIB_ENABLE_OBJC_INTEROP)

stdlib/cmake/modules/SwiftSource.cmake

@@ -244,6 +244,10 @@ function(_add_target_variant_swift_compile_flags
         "-target" "${SWIFT_SDK_${sdk}_ARCH_${arch}_TRIPLE}")
   endif()
 
+  if("${sdk}" STREQUAL "WASI")
+    list(APPEND result "-Xcc" "-D_WASI_EMULATED_MMAN" "-Xcc" "-D_WASI_EMULATED_SIGNAL" "-Xcc" "-D_WASI_EMULATED_PROCESS_CLOCKS")
+  endif()
+
   if(NOT BUILD_STANDALONE)
     list(APPEND result "-resource-dir" "${SWIFTLIB_DIR}")
   endif()