arm64: guard asm/assembler.h against multiple inclusions

asm/assembler.h lacks the usual guard against multiple inclusion,
leading to a compilation failure if it is accidentally included
twice.

Using the classic #ifndef/#define/#endif construct solves the issue.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>