AArch64: aarch64-asmtest.py may generate undefined register r18

Test script aarch64-asmtest.py may generate undefined register r18.

Error message:
Creating javadoc element list
/home/dongbo/jdk/src/hotspot/cpu/aarch64/assembler_aarch64.cpp: In function 'void entry(CodeBuffer*)':
/home/dongbo/jdk/src/hotspot/cpu/aarch64/assembler_aarch64.cpp:580:37: error: 'r18' was not declared in this scope; did you mean 'z18'?
  580 | __ ldp(r28, r22, Address(__ pre(r18, 16))); // ldp x28, x22, [x18, #16]!
      | ^~~
      | z18
gmake[3]: *** [lib/CompileJvm.gmk:143: /home/dongbo/jdk/build/linux-aarch64-server-fastdebug/hotspot/variant-server/libjvm/objs/assembler_aarch64.o] Error 1
gmake[3]: *** Waiting for unfinished jobs....
Compiling 89 properties into resource bundles for java.desktop