AArch64: Replace C initializers with memset

AArch64: Replace C initializers with memset

Clang doesn't accept {0} as a valid C struct initializer under their implementation
of -Wmissing-field-initializers.  This makes using C initializers a bit tricky.

Instead I'm changing the code to use memset instead, which at least GCC inlines and
generates the same code for.  This also seems to be the idiom used in binutils for
most targets.

opcodes/

	* aarch64-opc.c (verify_constraints): Use memset instead of {0}.