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/target/linux/jerry-asm.S

@@ -14,6 +14,12 @@ _start:
   _START
 .size _start, . - _start
 
+.global syscall_0_asm
+.type syscall_0_asm, %function
+syscall_0_asm:
+  SYSCALL_0
+.size syscall_0_asm, . - syscall_0_asm
+
 .global syscall_1_asm
 .type syscall_1_asm, %function
 syscall_1_asm: