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 can be used in the future to trigger garbage collection. For now,
it provides a more useful error message in case the heap is completely
filled up.
Make sure every to-be-implemented GC can use the same interface. As a
result, a 1MB chunk of RAM is allocated on Unix systems on init instead
of allocating on demand.
* Use 64-bit integers on 64-bit platforms, just like gc and gccgo:
https://golang.org/doc/go1.1#int
* Do not use a separate length type. Instead, use uintptr everywhere a
length is expected.
Let the standard library think that it is compiling for js/wasm.
The most correct way of supporting bare metal Cortex-M targets would be
using the 'arm' build tag and specifying no OS or an 'undefined' OS
(perhaps GOOS=noos?). However, there is no build tag for specifying no
OS at all, the closest possible is GOOS=js which makes very few
assumptions.
Sadly GOOS=js also makes some assumptions: it assumes to be running with
GOARCH=wasm. This would not be such a problem, just add js, wasm and arm
as build tags. However, having two GOARCH build tags leads to an error
in internal/cpu: it defines variables for both architectures which then
conflict.
To work around these problems, the 'arm' target has been renamed to
'tinygo.arm', which should work around these problems. In the future, a
GOOS=noos (or similar) should be added which can work with any
architecture and doesn't implement OS-specific stuff.
