mk: Add check-secondary target for tests with no x-platform risk.

We'll use this to run a subset of the test suite onto a dedicated
bot.

This puts the grammar tests and the pretty-printer tests under
check-secondary. It leanves the pretty tests under plain `check`
for now, until the new bot is added to take over.

Because check-secondary is not run as part of `make check` there
will be a set of tests that most users never run and are only
checked by bors. I think this will be ok because grammar tests
should rarely regress, and the people regressing such tests
should have the fortitude to deal with it.
mk: Add comments to primary check targets
mk: Some fixes to grammar.mk

Invoke rustc in a way that sets LD_LIBRARY_PATH,
and disable the deps on RustLexer.class when antlr isn't available.
mk: Remove check-syntax target

This appears to be redundantly named with a shortcut target for
testing the syntax crate.
mk: Print grammar info message like others
auto merge of #15988 : brson/rust/antlrprep, r=cmr

Add `check-secondary`, which runs the pretty and lexer tests so we can run them on a dedicated bot. Leaves the pretty tests running under `check` until the bots are switched over. Also fixes some issues.

r? @cmr