Add x86_64 kernel bring-up with build-selected platform adapters

src/arch/common/platform.process.inc

@@ -0,0 +1,15 @@
+// Copyright 2026 Aaron Hance
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+{ Shared contract; implemented by each build-selected adapter. }
+function prepare(stack: pointer; bytes: SizeUInt; entry: pointer): PtrUInt;
+function timerTicks: uint32;