Previously we used the i386 target, probably with all optional features
disabled. However, the Pentium 4 has been released a _long_ time ago and
it seems reasonable to me to take that as a minimum requirement.
Upstream Go now also seems to move in this direction:
https://github.com/golang/go/issues/40255
The main motivation for this is that there were floating point issues
when running the tests for the math package:
GOARCH=386 tinygo test math
I haven't investigated what's the issue, but I strongly suspect it's
caused by the weird x87 80-bit floating point format. This could perhaps
be fixed in a different way (by setting the FPU precision to 64 bits)
but I figured that just setting the minimum requirement to the Pentium 4
would probably be fine. If needed, we can respect the GO386 environment
variable to support these very old CPUs.
To support this newer CPU, I had to make sure that the stack is aligned
to 16 bytes everywhere. This was not yet always the case.
The only architecture that actually needs special support for scanning
the stack is WebAssembly. All others allow raw access to the stack with
a small bit of assembly. Therefore, don't manually keep track of all
these objects on the stack manually and instead just use conservative
stack scanning.
This results in a massive code size decrease in the affected targets
(only tested linux/amd64 for code size) - sometimes around 33%. It also
allows for future improvements such as using proper stackful goroutines.
