AArch64 Zero build is broken after JDK-8252684

Zero does not have AArch64 assembler, so attempt to use it from the test fails to build:

* For target hotspot_variant-zero_libjvm_gtest_objs_test_assembler_aarch64.o:
./test/hotspot/gtest/aarch64/test_assembler_aarch64.cpp: In member function 'virtual void AssemblerAArch64_validate_vm_Test::TestBody()':
./test/hotspot/gtest/aarch64/test_assembler_aarch64.cpp:57:13: error: cannot declare variable '_masm' to be of abstract type 'Assembler'
   Assembler _masm(&code);
             ^~~~~
In file included from ./src/hotspot/share/asm/assembler.hpp:451:0,
                 from ./test/hotspot/gtest/aarch64/test_assembler_aarch64.cpp:29:
./src/hotspot/cpu/zero/assembler_zero.hpp:34:7: note: because the following virtual functions are pure within 'Assembler':
 class Assembler : public AbstractAssembler {
       ^~~~~~~~~
In file included from ./test/hotspot/gtest/aarch64/test_assembler_aarch64.cpp:29:0:
./src/hotspot/share/asm/assembler.hpp:437:16: note: virtual void AbstractAssembler::bang_stack_with_offset(int)
   virtual void bang_stack_with_offset(int offset) = 0;
                ^~~~~~~~~~~~~~~~~~~~~~
In file included from ./test/hotspot/gtest/aarch64/test_assembler_aarch64.cpp:61:0:
./test/hotspot/gtest/aarch64/asmtest.out.h:7:8: error: 'class Assembler' has no member named 'add'
   ... (rest of output omitted)