Upstreaming Wasm32 target to Musl

[arjunr2]

[titzer]

1. All things related to floating point environment, fegetenv should be stubbed to nops on Wasm.

[titzer]

2. Definitions of FUTEX_WAIT and company were added to include/linux/futex.h in order to compile libcxx in LLVM. Not sure why these are not already defined.

[titzer]

3. Printing of long double (which maps to emulated fp128 on wasm32) should be ported over from Emscripten.

[titzer]

4. Stub out setjmp and longjmp in MUSL instead of WALI imports.

[titzer]

5. Pack epoll_data struct.

[titzer]

6. Define WALI trampolines for functions where the header is changed to be a WALI import.

[titzer]

7. Need to define a strategy for name-bound syscalls globally; could probably be factored to a single PR to do them for all calls globally.

[titzer]

8. __wasm_init_tls is an LLVM builtin and the patch redoes __copy_tls to forward to it.