Add abort () to jerry-libc

Added declaration and implementations of `void abort (void)` to
jerry-libc. As the linux implementation relies on the `getpid`
syscall - which has no arguments - `syscall_0` implementations
have been added as well.

`libc_fatal` has been adapted to use the new `abort` function
instead of `exit`. This also made the definition of
`LIBC_FATAL_ERROR_EXIT_CODE` unnecessary.

Finally, the syscall fatal error message was fixed.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-libc/jerry-libc-defs.h

@@ -28,11 +28,6 @@
 #define __attr_noreturn___ __attribute__((noreturn))
 #define __attr_noinline___ __attribute__((noinline))
 
-/**
- * Constants
- */
-#define LIBC_FATAL_ERROR_EXIT_CODE (2)
-
 /**
  * Assertions
  */