Set unknown_syscall differently on arm linux

Currently, we use 123456789 as unknown or illegal syscall number, and
expect program return ENOSYS.  Although 123456789 is an illegal syscall
number on arm linux, kernel sends SIGILL rather than returns -ENOSYS.
However, arm linux kernel returns -ENOSYS if syscall number is within
0xf0001..0xf07ff, so we can use 0xf07ff for unknown_syscall in test.

gdb/testsuite:

2016-06-29  Yao Qi  <yao.qi@linaro.org>

	* gdb.base/catch-syscall.c [__arm__]: Set unknown_syscall to
	0x0f07ff.