Autogenerate syscall table based on other files

[clemdiep]

This PR make it easier to maintain the syscall table used to match syscall number to their corresponding function.

For that, this script syscall_gen.sh is added. It will take as input the new file syscall.tbl and arch/ARCH/syscall.h.in and generate the files include/generated/syscall_number.h and include/generated/syscall_table.h.in.
This script is called by the build system in the first steps.

syscall.tbl is a custom file which list all syscall available in SO3 and a configuration requirements if there is any. This file isn't arch dependent and all new syscall implementation needs to be added there.

arch/ARCH/syscall.h.in is an exact copy from musl arch/ARCH/bits/syscall.h.in which list all syscall available on Linux why there corresponding number.

include/generated/syscall_number.h will contain the all syscalls that are listed in both syscall.tbl and syscall.h.in, with #ifdef CONFIG_... around if necessary. This means that now if SYSYCALL_xxx is defined in the kernel, then the syscall is activated. It can allow to remove undeed syscall implementation to reduce the kernel code size.

include/generated/syscall_table.h.in will contain all syscall from syscall.tbl in the following format:

#ifdef SYSCALL_read
	[SYSCALL_read] = &__sys_read,
#endif

This file can then be included in the syscall table initialisation list to get the correct list of used syscall function with their correct number.

Those changes allow for a more easily maintainable list of syscall as only syscall.tbl needs to be updated when implmenting a new one, without having to add a lot of #if around syscall table elements.

[clemdiep]

In draft as #211 should be merge before to avoid conflict.
However, it may already be reviewed.