Cirrus-CI: Add manual Clang 18 jobs

Work is in progress to import Clang/LLVM 18 as the base system compiler.
llvm18-lite is not yet in the quarterly package set but but will be
available in the not too distant future, and is available for src forks
that switch to latest packages.  Add manual llvm18 jobs now so that they
can be used for testing.

Sponsored by:	The FreeBSD Foundation

.cirrus.yml

@@ -29,6 +29,14 @@ task:
       TARGET_ARCH: amd64
       TOOLCHAIN: llvm17
       TOOLCHAIN_PKG: ${TOOLCHAIN}-lite
+  - name: amd64-llvm18 World and kernel build and boot smoke test
+    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
+    trigger_type: manual
+    env:
+      TARGET: amd64
+      TARGET_ARCH: amd64
+      TOOLCHAIN: llvm18
+      TOOLCHAIN_PKG: ${TOOLCHAIN}-lite
   - name: arm64-llvm16 World and kernel build and boot smoke test
     only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
     trigger_type: manual
@@ -45,6 +53,14 @@ task:
       TARGET_ARCH: aarch64
       TOOLCHAIN: llvm17
       TOOLCHAIN_PKG: ${TOOLCHAIN}
+  - name: arm64-llvm18 World and kernel build and boot smoke test
+    only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
+    trigger_type: manual
+    env:
+      TARGET: arm64
+      TARGET_ARCH: aarch64
+      TOOLCHAIN: llvm18
+      TOOLCHAIN_PKG: ${TOOLCHAIN}
   - name: amd64-gcc12 World and kernel build and boot smoke test (manual)
     only_if: $CIRRUS_REPO_FULL_NAME != 'freebsd/freebsd-src'
     trigger_type: manual