Add signed to unsigned arithmetic

[anoushk1234]

When converting from a signed integer to unsigned integer the value should be 2^(bits)-value

Ex:
signed 32 bit: -1
unsigned 32 bit casted: 2^32 - 1

[anoushk1234]

Currently sets to max but this is wrong:

zig-ebpf/src/interpreter.zig

Lines 238 to 242 in d294476

	if (ix.imm >= 0) {
	reg[dst] = @as(u64, @intCast(ix.imm));
	} else {
	reg[dst] = std.math.maxInt(u64);
	}