Fix GitHub Actions config (actually run gdb tests on new backend).

I had missed that the CI config didn't actually run the tests, because
(I think) `matrix.target` is not set by default (?). All of our hosts
are native x86-64, so we can just gate on OS (Ubuntu) instead.

I also discovered that while I had been testing with the gdb tests
locally, when *all* `debug::*` tests are run, there are two that do not
pass on the new backend because of specific differences in compiled
code. One is a value-lifetime issue (the value is "optimized out" at the
point the breakpoint is set) and the other has to do with basic-block
order (it is trying to match against hardcoded machine-code offsets
which have changed).

.github/workflows/main.yml

@@ -295,8 +295,8 @@ jobs:
     # Test debug (DWARF) related functionality on new backend.
     - run: |
         sudo apt-get update && sudo apt-get install -y gdb
-        cargo test --features experimental_x64 test_debug_dwarf -- --ignored --test-threads 1
+        cargo test --features experimental_x64 test_debug_dwarf -- --ignored --test-threads 1 --test debug::
-      if: matrix.target == 'x86_64-unknown-linux-gnu'
+      if: matrix.os == 'ubuntu-latest'
       env:
         RUST_BACKTRACE: 1
 

tests/all/debug/lldb.rs

@@ -137,7 +137,11 @@ check: exited with status
 #[ignore]
 #[cfg(all(
     any(target_os = "linux", target_os = "macos"),
-    target_pointer_width = "64"
+    target_pointer_width = "64",
+    // Ignore test on new backend. The value this is looking for is
+    // not available at the point that the breakpoint is set when
+    // compiled by the new backend.
+    not(feature = "experimental_x64"),
 ))]
 pub fn test_debug_dwarf_ptr() -> Result<()> {
     let output = lldb_with_script(

tests/all/debug/translate.rs

@@ -114,7 +114,11 @@ check:        DW_AT_decl_line	(10)
 #[cfg(all(
     any(target_os = "linux", target_os = "macos"),
     target_arch = "x86_64",
-    target_pointer_width = "64"
+    target_pointer_width = "64",
+    // Ignore test on new backend. This is a specific test with hardcoded
+    // offsets and the new backend compiles the return basic-block at a different
+    // offset, causing mismatches.
+    not(feature = "experimental_x64"),
 ))]
 fn test_debug_dwarf5_translate_lines() -> Result<()> {
     check_line_program(