aarch64: Use gdb_sys_no_syscall enum instead of -1

aarch64: Use gdb_sys_no_syscall enum instead of -1

Fixes:

/home/simark/src/binutils-gdb/gdb/aarch64-linux-tdep.c: In function "gdb_syscall aarch64_canonicalize_syscall(aarch64_syscall)":
/home/simark/src/binutils-gdb/gdb/aarch64-linux-tdep.c:874:13: error: invalid conversion from "int" to "gdb_syscall" [-fpermissive]
     return -1;
             ^

gdb/ChangeLog:

	* aarch64-linux-tdep.c (aarch64_canonicalize_syscall): Return
	gdb_sys_no_syscall instead of -1.