arm64: Do not forget syscall when starting a new thread.

Enable tracing of the execve*() system calls with the
syscalls:sys_exit_execve tracepoint by removing the call to
forget_syscall() when starting a new thread and preserving the value of
regs->syscallno across exec.
Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>
Link: https://lore.kernel.org/r/20220608162447.666494-2-flaniel@linux.microsoft.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/processor.h

@@ -272,8 +272,9 @@ void tls_preserve_current_state(void);
 
 static inline void start_thread_common(struct pt_regs *regs, unsigned long pc)
 {
+	s32 previous_syscall = regs->syscallno;
 	memset(regs, 0, sizeof(*regs));
-	forget_syscall(regs);
+	regs->syscallno = previous_syscall;
 	regs->pc = pc;
 
 	if (system_uses_irq_prio_masking())