LLDB mostly works on most platforms, but it is still lacking in some
features. For example, it doesn't seem to support RISC-V yet (coming in
LLVM 12), it only partially supports AVR (no stacktraces), and it
doesn't seem to support the Ctrl-C keyboard command when running a
binary for another platform (e.g. with GOOS=arm64). However, it does
mostly work, even on baremetal systems.
This commit also adds a bit of version independence, in particular for
external commands. It also adds the LLVM version to the `tinygo version`
command, which might help while debugging.
This is a large commit that moves all code directly related to
compiling/linking into a new builder package. This has a number of
advantages:
* It cleanly separates the API between the command line and the full
compilation (with a very small API surface).
* When the compiler finally compiles one package at a time (instead of
everything at once as it does now), something will have to invoke it
once per package. This builder package will be the natural place to
do that, and also be the place where the whole process can be
parallelized.
* It allows the TinyGo compiler to be used as a package. A client can
simply import the builder package and compile code using it.
As part of this refactor, the following additional things changed:
* Exported symbols have been made unexported when they weren't needed.
* The compilation target has been moved into the compileopts.Options
struct. This is done because the target really is just another
compiler option, and the API is simplified by moving it in there.
* The moveFile function has been duplicated. It does not really belong
in the builder API but is used both by the builder and the command
line. Moving it into a separate package didn't seem useful either
for what is essentially an utility function.
* Some doc strings have been improved.
Some future changes/refactors I'd like to make after this commit:
* Clean up the API between the builder and the compiler package.
* Perhaps move the test files (in testdata/) into the builder package.
* Perhaps move the loader package into the builder package.
