Fix AArch64 OpenJDK build failure with gcc-5

OpenJDK build failed on AArch64 with gcc-5:

---
For target hotspot_variant-server_libjvm_objs_vm_version_linux_aarch64.o:
./src/hotspot/os_cpu/linux_aarch64/vm_version_linux_aarch64.cpp: In static member function 'static void VM_Version::get_os_cpu_info()':
./src/hotspot/os_cpu/linux_aarch64/vm_version_linux_aarch64.cpp:96:40: error: expected ',' before ')' token
static_assert(CPU_FP == HWCAP_FP);
^
./src/hotspot/os_cpu/linux_aarch64/vm_version_linux_aarch64.cpp:96:40: error: expected string-literal before ')' token
./src/hotspot/os_cpu/linux_aarch64/vm_version_linux_aarch64.cpp:97:43: error: expected ',' before ')' token
static_assert(CPU_ASIMD == HWCAP_ASIMD);
^

As one C++17 feature, the argument 'message' of static_assert can be optional. This feature, i.e. static_assert with no message, is supported by gcc-6 or higher.[1]