configs: Add FTRACE_SYSCALLS=y

[pelwell]

FTRACE_SYSCALLS enables the syscall entry and exit events to be traced.

See: #6752

[pelwell]

This option adds 170kB to the uncompressed ARMv7 kernel - the one which is most size sensitive, since the memories are smallest:

Build	File	size
Before	Image	16229536
	kernel.img	7316892
After	Image	16408704
	kernel.img	7371224

Are you happy with this, @popcornmix?

[popcornmix]

The size increase is more than I expected. Perhaps this needs more thought to see exactly how many users it would benefit.

I did ask a colleague who has been using perfetto for tracing system activity if this would be useful and he says perfetto already supports tracing syscalls using these events:

$ sudo cat /sys/kernel/debug/tracing/available_events | grep syscalls
raw_syscalls:sys_exit
raw_syscalls:sys_enter