arm64: Ensure that the 'bti' macro is defined where linkage.h is included

Not all .S files include asm/assembler.h, however the SYM_FUNC_*
definitions invoke the 'bti' macro. Include asm/assembler.h in
asm/linkage.h.

Fixes: 9be34be8 ("arm64: Add macro version of the BTI instruction")
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>