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/asm_x64.h

@@ -16,6 +16,15 @@
 #ifndef ASM_X64_H
 #define ASM_X64_H
 
+/*
+ * mov syscall_no (%rdi) -> %rax
+ * syscall
+ */
+#define SYSCALL_0 \
+  mov %rdi, %rax; \
+  syscall; \
+  ret;
+
 /*
  * mov syscall_no (%rdi) -> %rax
  * mov arg1 (%rsi) -> %rdi