AArch64: Detect exit from execve syscall

AArch64: Detect exit from execve syscall

Checking the syscall number when stopped on entry/exit relies on checking
the value in register X8.

However, on exit from an execve syscall, the registers will all be cleared.
Given this is only checked on syscall entry/exit, then a cleared register
state either means execve exit or syscall 0 (io_setup) entry with invalid
parameters and an invalid FR and LR, which in reality should never happen.
Use this to detect execve exit.

Move function to allow use of aarch64_sys_execve enum, and use newer
regcache functions.

Fixes gdb.base/catch-syscall.exp on Aarch64.

gdb/ChangeLog:

	* aarch64-linux-tdep.c (aarch64_linux_get_syscall_number): Check
	for execve.