[SOL] Implement syscall instruction

[LucasSte]

SIMD-0178 introduces a syscall instruction which comprises the opcode 0x95 and an immediate value. The value is supposed to be an integer greater than zero, representing the syscall we want to invoke.

Following the external call scheme on BPF, we'll consider a syscall any call to a static address.

The reason behind this design was to avoid compile work when introducing of a syscall. Identifying syscalls by their symbol name would required either a hard coded lookup table or loading an external file. The former case would require a toolchain recompilation and the latter a new platform tools release simply to add a syscall, increasing the work overhead.

The front-end must specify the syscall header as a constant function pointer and assign to it the corresponding syscall number. This is similar to existing scheme for static syscalls, except that we won't be using the murmur32 hash anymore. Syscalls should be declared as the following example:

int (*const syscall)(int a, int b) = (void*)(syscall_number);

[LucasSte]

In some circumstances, specially when we build the code with -O0, the front end may emit a callx. Instead of creating a peephole or forcing optimizations in the backend, I decided to add the correct declaration example in both C and Rust.

These examples generate LLVM-IR code that even when unoptimized will create a syscall instruction.