[GDBServer][AArch64] Use the same break instruction as GDB

[GDBServer][AArch64] Use the same break instruction as GDB

GDB uses a "brk #0" instruction to perform a software breakpoint while
GDBServer uses an illegal instruction.  Both instructions should match.

When enabling support for the 'Z0' packet, we let GDBServer insert the
breakpoint instruction instead of GDB.  And in case of permanent
breakpoints for example, GDB will check if a breakpoint is inserted in the
inferior with `program_breakpoint_here_p (gdbarch, address)', and
compare the instruction read from the inferior with the breakpoint
instruction.

On AArch64, instructions are always little endian so we need to
represent it as an array of bytes, as done in aarch64-tdep.c.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c: Remove comment about endianness.
	(aarch64_breakpoint): Change type to gdb_byte[].  Set to "brk #0".
	(aarch64_breakpoint_at): Change type of insn to gdb_byte[].  Use
	memcmp.