When we're compiling compiler_rt for any WebAssembly target, we do
not want to expose all the compiler-rt functions to the host runtime.
By setting the visibility of all exports to `hidden`, we allow the
linker to resolve the symbols during linktime, while not expose the
functions to the host runtime. This also means the linker can
properly garbage collect any compiler-rt function that does not get
resolved. The symbol visibility for all target remains the same as
before: `default`.
This moves functions that LLVM generates calls to,
to the compiler_rt implementation itself, rather than c.zig.
This is a prerequisite for native backends to link with compiler-rt.
This also allows native backends to generate calls to `memcpy` and the like.
