Added #if control around GCC built-in functions so that the code can be (#1483)

compiled using non-GCC compilers.

This included adding an initialization to a variable that looks(!)
like it can reach a use before being initialized (b/c we turned
JERRY_UNREACHABLE into a nop) -- an example:

int foo;
switch (value_can_only_be_one_or_two)
   case 1:
   case 2:
       foo = 5;
   default:
      JERRY_UNREACHABLE();
x = foo +1;

...the compiler assumes that the path can go through the default case,
which leaves the value of foo undefined.

JerryScript-DCO-1.0-Signed-off-by: Timothy Harvey t-harvey@ti.com

jerry-libc/include/assert.h

@@ -28,7 +28,7 @@ extern "C"
 #define assert(x) \
   do \
   { \
-    if (__builtin_expect (!(x), 0)) \
+    if (!(x)) \
     { \
       fprintf (stderr, "%s:%d: %s: Assertion `%s' failed.", __FILE__, __LINE__, __func__, #x); \
       abort (); \