Set up XRAM-related compiler options

Last time we looked at the default startup code and found that it’s not
very suitable for our architecture, because it assumes that access to
high XRAM addresses is done by writing the high byte of the address into
P2.

SDCC has two options that are related to XRAM:

* `--no-xinit-opt` disables the optimization of initialized variables.
  If it is enabled (by default), the initial values are stored in a part
  of code and copied into XRAM on startup. If disabled, the code will
  contain actual MOV instructions to initialize each variable.
* `--xram-size` tells the compiler how much XRAM we have. It can then
  warn us if we define more variables than can fit.