Changes to WASI spec in snapshot_preview1

[cjihrig]

Changes that need to be addressed in uvwasi

• The whence_t values changed in Change whence_t constant values to match pre-existing agreed-upon values. WebAssembly/WASI#106. This change landed in ephemeral, not unstable.
• The __wasi_linkcount_t data type changed to uint64_t in Change hard link count to 64-bits. WebAssembly/WASI#127. This change landed in ephemeral only.
• The subscription_t's clock.identifier field was removed in Remove subscription_t's clock.identifier field. WebAssembly/WASI#125. This change landed in ephemeral only.
• The __wasi_proc_raise() function and the __wasi_signal_t type and constants are being removed in Remove proc_raise and the signal types and constants. WebAssembly/WASI#136. EDIT: This change impacts ephemeral only. This change isn't in snapshot_preview1

Changes that have been addressed in uvwasi

• The clockid_t values changed in Reorder the clockid_t values to match existing practice. WebAssembly/WASI#112. This change langed in ephemeral, as well as wasi_unstable_preview0. (Addressed in sync constants from wasi-libc #47)