Roll wasi-libc to include snapshot1 (WebAssembly#84)

This brings in the following changes:

f645f49 Update signal macros after upgrade to snapshot1 (WebAssembly#144)
8b3266d github actions: pin checkout action to v1 (WebAssembly#145)
410c660 Use constructor functions for optional init routines. (WebAssembly#142)
fe13053 c header generation updated for reorganized witx ast (WebAssembly#139)
cd74e1d Correct the version of WebAssembly#136 on master (WebAssembly#141)
446cb3f Wasi snapshot preview1 (WebAssembly#140)
54102f0 Ignore rights in libpreopen. (WebAssembly#129)
8c9e1c6 Make the `__original_main` definition weak, fixing -flto. (WebAssembly#138)
cf81683 Optimize `fmin`, `fmax`, etc. (WebAssembly#120)
deb8eae Don't pre-check capabilities in `openat`. (WebAssembly#130)
ca9046d Use consistent style for wasi-libc C source files. (WebAssembly#131)
951cc3e Fix unintended recursion in __wasilibc_register_preopened_fd. (WebAssembly#133)
5216983 Avoid a `strdup` call in `__wasilibc_populate_libpreopen`. (WebAssembly#128)
70099d4 Don't link in libpreopen initialization code when it isn't needed. (WebAssembly#127)
ec4549d Temporarily disable the use of `__heap_base`. (WebAssembly#132)
a214f1c Use __heap_base by dlmalloc (WebAssembly#114)
a94d2d0 Avoid varargs conventions when calling open (WebAssembly#126)
7fcc4f2 Revamp and simplify the libpreopen code. (WebAssembly#110)
eb7230c Remove more unsupported headers. (WebAssembly#123)