Rename thread_spawn import (WebAssembly#387)

Following the wit-defined ABI:
WebAssembly/wasi-threads#26

Author: yamt

expected/wasm64-wasi/predefined-macros.txt

@@ -5409,6 +5409,7 @@
 #define __DEFINED_wctype_t 
 #define __DEFINED_wint_t 
 #define __FINITE_MATH_ONLY__ 0
+#define __FLOAT128__ 1
 #define __FLT(x) (__IS_REAL(x) && sizeof(x) == sizeof(float))
 #define __FLTCX(x) (__IS_CX(x) && sizeof(x) == sizeof(float complex))
 #define __FLT_DECIMAL_DIG__ 9