Update PR tests slightly and when what runs where (#5805)

* Don't run LLDB tests on PRs

These take an extra minute or so, so only run them on the full test
suite of a merge instead of on all PRs as well.

* Add a test for the x64 isa files

This guarantees that if cranelift's x64 backend is modified that the
tests will be run on a PR, even if other backends were also modified.

ci/build-test-matrix.js

@@ -36,7 +36,8 @@ const array = [
   {
     "os": "ubuntu-latest",
     "name": "Test Linux x86_64",
-    "filter": "linux-x64"
+    "filter": "linux-x64",
+    "isa": "x64"
   },
   {
     "os": "macos-latest",