Introducing several emscripten-specific changes to the llvm-libc headers
as well as adding a wasm sigsetjmp.cpp.

Towards the goal of getting `ninja libc-lint` back to green, fix the numerous
instances of:

    warning: header guard does not follow preferred style [llvm-header-guard]

This is because many of our header guards start with `__LLVM` rather than
`LLVM`.

To filter just these warnings:

    $ ninja -k2000 libc-lint 2>&1 | grep llvm-header-guard

To automatically apply fixits:

    $ find libc/src libc/include libc/test -name \*.h | \
        xargs -n1 -I {} clang-tidy {} -p build/compile_commands.json \
        -checks='-*,llvm-header-guard' --fix --quiet

Some manual cleanup is still necessary as headers that were missing header
guards outright will have them inserted before the license block (we prefer
them after).

This is largely a cosmetic change done with a few goals:
1. Reduce the conditionals in picking the correct set of tables for the
   platform.
2. Avoid exposing, for example Linux errors, when building for non-Linux
   platforms. This also prevents build failures when Linux errors are not
   defined on the target non-Linux platform.
3. Some "_table" suffixes have been removed to avoid repeated
   occurance of "table" like "tables/linux_error_table.h".

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D151367