This patch fixes the following failure:
FAIL: gdb.trace/trace-condition.exp: ftrace: -(21 << 1) == -42: check 10
frames were collected.

This was due to aarch64_emit_sub using the wrong order in its operands, so the
operation would end up being 42 - 0 rather than 0 - 42.

This patch also fixes the order of aarch64_emit_add for clarity.

The test case for emit_sub is fixed so that the proper order of
the operands is needed for the test to pass.

Tested on aarch64-native-extended-gdbserver.

Note: trace-condition.exp was broken a bit so I had to modify it to run
the test. A fix is coming for that in another patch.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c (aarch64_emit_add): Switch x1 and x0.
	(aarch64_emit_sub): Likewise.

gdb/testsuite/ChangeLog:

	* gdb.trace/trace-condition.exp (foreach): Fix emit_sub testcase.