use architecture-specific ssize_t definition

after change 32822 was rejected, this is the more light-weight
version of the fix: libc/include/sys/types.h already - via
libc/kernel/common/linux/posix_types.h - includes a definition
of __kernel_ssize_t from libc/kernel/arch-*/asm/posix_types.h
which is architecture-specific, toolchain-agnostic and also
gets rid of the gcc -Wformat warning (which it issues correctly,
since this i̲s̲ indeed a bug in bionic)

Change-Id: Ie4503ab16628bc25815a836d07556f665e9795c7
2 files changed