arm/syscalls: mark syscall invocation as likely in invoke_syscall

The invoke_syscall() function is overwhelmingly called for
valid system call entries. Annotate the main path with likely()
to help the compiler generate better branch prediction hints,
reducing CPU pipeline stalls due to mispredictions.

This is a micro-optimization targeting syscall-heavy workloads [1].

Link: https://lore.kernel.org/r/20250922121730.986761-1-pengcan@kylinos.cn

 [1]
Signed-off-by: Can Peng <pengcan@kylinos.cn>
Signed-off-by: Will Deacon <will@kernel.org>