feat(tvix/eval): implement builtins.currentSystem r/5190

This commit implements builtins.currentSystem, by capturing the
cargo environment variable `TARGET` and exposing it to rustc as
`TVIX_CURRENT_SYSTEM` so it can be inserted into the source code
using `env!()`.

The resulting value needs to be massaged a bit, since it is an "LLVM
triple".  The current code should work for all the platforms for
which cppnix works (thanks qyliss for generating the list!).  It
does *not* reject all of the triples that cppnix's configure.ac
rejects -- it is much more forgiving.  We can tighten this up in a
future commit.

Signed-off-by: Adam Joseph <adam@westernsemico.com>
Change-Id: I947f504b2af5a7fee8cf0cb301421d2fc9174ce1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6986
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

0 files changed, 0 insertions, 0 deletions