Use LLVM libc ssize_t.h

There is no sys/types.h header in LLVM libc for baremetal targets as it
is a POSIX one. ssize_t however is provided via ssize_t.h. Conditionally
include this header when LLVM Libc is used as that's the reason why
sys/types.h is included in the first place which is POSIX only.

Author: Prabhuk

include/cxx.h

@@ -16,6 +16,8 @@
 #include <vector>
 #if defined(_WIN32)
 #include <basetsd.h>
+#elif defined(__LLVM_LIBC__)
+#include <llvm-libc-types/ssize_t.h>
 #else
 #include <sys/types.h>
 #endif