amd64: annotate the syscall return address check with __predict_false

amd64: annotate the syscall return address check with __predict_false

before:
   0xffffffff80b03ebb <+2059>:	mov    0x460(%r14),%rax
   0xffffffff80b03ec2 <+2066>:	mov    0x98(%rax),%rax
   0xffffffff80b03ec9 <+2073>:	shr    $0x2f,%rax
   0xffffffff80b03ecd <+2077>:	je     0xffffffff80b03edd <amd64_syscall+2093>
   0xffffffff80b03ecf <+2079>:	mov    0x3f8(%r14),%rax
   0xffffffff80b03ed6 <+2086>:	orl    $0x1,0xc8(%rax)
   0xffffffff80b03edd <+2093>:	add    $0xf8,%rsp

after:
   0xffffffff80b03ebb <+2059>:	mov    0x460(%r14),%rax
   0xffffffff80b03ec2 <+2066>:	mov    0x98(%rax),%rax
   0xffffffff80b03ec9 <+2073>:	shr    $0x2f,%rax
   0xffffffff80b03ecd <+2077>:	jne    0xffffffff80b03eef <amd64_syscall+2111>
   0xffffffff80b03ecf <+2079>:	add    $0xf8,%rsp

Reviewed by:	kib
MFC after:	1 week