feat(lint): flag writes to the platform-reserved register

lint/lint.go

@@ -81,6 +81,7 @@ const (
 	CodeABI0RegisterArgs      = "abi0-register-args"
 	CodeNonportableRegister   = "nonportable-register-name"
 	CodeUnencodable           = "unencodable-instruction"
+	CodeReservedRegister      = "reserved-register-write"
 )
 
 // knownTextFlags are the flags recognised by the Go assembler's textflag.h.
@@ -469,6 +470,12 @@ func lintText(t *ast.Text, tab *arch.Table, archKnown bool, cfg Config, macros m
 		out = append(out, scanNonportableRegisters(t)...)
 	}
 
+	// Writes to the platform-reserved register (arm64 R18), which the ABI
+	// checks cannot observe at runtime.
+	if cfg.Arch == arch.ARM64 && !hasMacro && !cfg.Disable[CodeReservedRegister] {
+		out = append(out, scanReservedRegisterWrites(t, cfg.Arch)...)
+	}
+
 	// FUNCDATA / PCDATA structural validation.
 	out = append(out, checkFuncdata(t, cfg)...)