ci: the build tool has no release for the other architecture, so reach it another way

Every job that asked for an Intel runner failed at the first step: the
installer answers "Unsupported release target: macosx-x86_64", so the suite
cannot be built there at all. That is a property of the tooling and not of this
implementation, and a row that always fails says nothing about either.

What can be done there is done instead. The `numbers' workflow reads that
kernel's own table on both architectures and issues every call this
implementation makes on both, and the conformance job now compiles every source
for the other architecture with the system's own compiler. The three things that
differ between the two --- the calling convention, the register the current
context is reached through, and one field of a signal context --- are all in
src/sys.h, and all three are compiled.

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -35,11 +35,14 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # arm64 and x86_64. The system-call numbers agree; the calling
-          # convention, the register the current context is reached through and
-          # one field of a signal context do not, so both are built.
+          # arm64 only, and not because the other architecture is unsupported.
+          # The build tool has no release for x86_64 on this system --- its
+          # installer answers "Unsupported release target: macosx-x86_64" --- so
+          # the suite cannot be built there at all. What can be done there is
+          # done: the `numbers' workflow measures that kernel's own table on
+          # both architectures, and the step below compiles every source for the
+          # other one with the system's compiler.
           - { os: macos-14, toolchain: 'llvm@20.1.7' }
-          - { os: macos-15-intel, toolchain: 'llvm@20.1.7' }
     steps:
       - uses: actions/checkout@v4
 
@@ -88,6 +91,22 @@ jobs:
         run: |
           bash .spec/tools/run-conformance.sh openkal-macos . full
 
+      # The other architecture, as far as this system allows it to be reached.
+      #
+      # The system-call numbers agree between the two --- measured, in the
+      # `numbers' workflow --- and three things do not: the calling convention,
+      # the register the current context is reached through, and one field of a
+      # signal context. All three are in src/sys.h and all three are compiled
+      # here, with the system's own compiler rather than the build tool's,
+      # because the build tool has no release for that architecture.
+      - name: Every source compiles for the other architecture
+        run: |
+          for f in src/*.cpp; do
+            clang++ -std=c++2b -c -o /dev/null --target=x86_64-apple-macos13 \
+              -fno-exceptions -fno-rtti -DOKM_STANDALONE -I .spec/include "$f"
+          done
+          echo "every source compiles for x86_64"
+
       - name: The exported surface is complete and contains nothing else
         run: |
           # Clause 9.3. The list and the checker come from the specification