Fix gold build error at -O0.

Fix gold build error at -O0.

In aarch64.cc, Erratum_stub::STUB_ADDR_ALIGN and Reloc_stub::STUB_ADDR_ALIGN
are declared as static const int, and initialized in the class body. These
values are referenced from outside the class body, and with no optimization,
the references go unresolved.

gold/
	* aarch64.cc (Erratum_stub::STUB_ADDR_ALIGN): Move initialization
	outside class body.
	(Reloc_stub::STUB_ADDR_ALIGN): Likewise.