Use a portable isize definition.

[Prabhuk]

Remove platform specific headers which were included for ssize_t and use intptr_t instead to prevent failures in baremetal builds.

[dtolnay]

Do you have an idea how to preserve this assertion in the cases that ssize_t is available?

cxx/src/cxx.cc

Lines 445 to 448 in cd508fc

	static_assert(sizeof(rust::isize) == sizeof(std::intptr_t),
	"unsupported ssize_t size");
	static_assert(alignof(rust::isize) == alignof(std::intptr_t),
	"unsupported ssize_t alignment");