This reverts commit fa445d86a110f1171b75824fe5ec139089fa4733.
Narrator: It did, in fact, make a difference.
For whatever reason, building LLVM against spacemit_x60 or baseline makes no
noticeable difference in terms of performance, but building the Zig compiler
against spacemit_x60 does. Also, the miscompilation that was causing
riscv64-linux-debug to fail was in the LLVM libraries, not in the Zig compiler,
so we may as well take the win here.
The idea is to have 2 runners per machine, since a lot of time is spent building
stage3 and stage4, both of which are largely single-core affairs. This will make
the test steps take longer, however, so the timeouts have been bumped a bit, and
max RSS for the test step has been lowered from 64G to 32G to prevent OOM.
Finally, we now only run a single ReleaseSafe job on PRs; Debug and Release jobs
are limited to pushes.
