Conditionalize use of POSIX features missing on WASI/WebAssembly.

This patch makes it possible to build LLVM, Clang, and LLD for
WASI/WebAssembly. This patch introduces conditionals of the form
`defined(__wasi__)` or `defined(__wasm__)` wherever necessary to detect
the use of the WASI platform. In addition, it introduces a `HAVE_SETJMP`
feature test macro because the WASI platform can have or lack support
for this feature depending on compiler options.