wirish/syscalls.c: Replace obsolete caddr_t with void *

caddr_t is obsolete in POSIX and thus unavailable in most modern
toolchains. caddr_t usage should be replaced by void *.

stddef.h includes the size_t typedef which was missing.
build-rules.mk: Add CROSS_COMPILE env var support

Since toolchains other then older Code Sourcery (Mentor Graphics)
might not use the arm-none-eabi triplet, make it possible to use
any triplet by just setting the CROSS_COMPILE environment variable
accordingly.