Simplifies the logic, clarifies the comment, and fixes a minor bug,
which is that we exported the Windows ABI name *instead* of the standard
compiler-rt name, but it's meant to be exported *in addition* to the
standard name (this is LLVM's behavior and it is more useful).
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`.
The purpose of this branch is to switch to using an object file for each
independent function, in order to make linking simpler - instead of
relying on `-ffunction-sections` and `--gc-sections`, which involves the
linker doing the work of linking everything and then undoing work via
garbage collection, this will allow the linker to only include the
compilation units that are depended on in the first place.
This commit makes progress towards that goal.
