xvargs: variadic arguments for x86_64

about:
	xvargs is a simple library non-thread safe from which you can get variadic
	arguments for x86_64 by pushing values into the stack before calling the
	desired function.

	DXVARG_SAFE macro: makes sure the program never produces a seg fault by
	reading forbidden data

	xvargs_init function: inits the xvargs library, if DXVARG_SAFE is not defined
	the function does not take arguments, else it will take two: rdi will specify
	the maximum number of arguments allowed to read, rsi will specify a default
	value in case the limit is reached

	xvargs_next function: returns the next element in the stack (next argument)

	xvargs_fini function: finishes the process and cleans internal data

notes:
	since arguments are pushed onto the stack, the arguments need to be pushed in
	the reverse order they will be used in the desired function