Go 1.12 switched to using libSystem.dylib for system calls, because
Apple recommends against doing direct system calls that Go 1.11 and
earlier did. For more information, see:
https://github.com/golang/go/issues/17490https://developer.apple.com/library/archive/qa/qa1118/_index.html
While the old syscall package was relatively easy to support in TinyGo
(just implement syscall.Syscall*), this got a whole lot harder with Go
1.12 as all syscalls now go through CGo magic to call the underlying
libSystem functions. Therefore, this commit overrides the stdlib syscall
package with a custom package that performs calls with libc (libSystem).
This may be useful not just for darwin but for other platforms as well
that do not place the stable ABI at the syscall boundary like Linux but
at the libc boundary.
Only a very minimal part of the syscall package has been implemented, to
get the tests to pass. More calls can easily be added in the future.
So far, we've pretended to be js/wasm in baremetal targets to make the
stdlib happy. Unfortunately, this has various problems because
syscall/js (a dependency of many stdlib packages) thinks it can do JS
calls, and emulating them gets quite hard with all changes to the
syscall/js packages in Go 1.12.
This commit does a few things:
* It lets baremetal targets pretend to be linux/arm instead of
js/wasm.
* It lets the loader only select particular packages from the src
overlay, instead of inserting them just before GOROOT. This makes it
possible to pick which packages to overlay for a given target.
* It adds a baremetal-only syscall package that stubs out almost all
syscalls.
This mostly works around the default CGo support: this compiler doesn't
need special wrappers for C functions as it uses the C calling
convention by default.
