This commit brings in a number of minor updates for rustc's support for
the wasm target which has changed in the LLVM 9 update. Notable updates
include:
* The compiler now no longer manually inserts the `producers` section,
instead relying on LLVM to do so. LLVM uses the `llvm.ident` metadata
for the `processed-by` directive (which is now emitted on the wasm
target in this PR) and it uses debuginfo to figure out what `language`
to put in the `producers` section.
* Threaded WebAssembly code now requires different flags to be passed
with LLD. In LLD we now pass:
* `--shared-memory` - required since objects are compiled with
atomics. This also means that the generated memory will be marked as
`shared`.
* `--max-memory=1GB` - required with the `--shared-memory` argument
since shared memories in WebAssembly must have a maximum size. The
1GB number is intended to be a conservative estimate for rustc, but
it should be overridable with `-C link-arg` if necessary.
* `--passive-segments` - this has become the default for multithreaded
memory, but when compiling a threaded module all data segments need
to be marked as passive to ensure they don't re-initialize memory
for each thread. This will also cause LLD to emit a synthetic
function to initialize memory which users will have to arrange to
call.
* The `__heap_base` and `__data_end` globals are explicitly exported
since they're now hidden by default due to the `--export` flags we
pass to LLD.
