Add jerryscript-compiler.h public header to cover compiler incompatibilities (#2313)

In general, public headers should not have compiler-specific
constructs but both the core and the port headers have attributes,
which are non-standard. It's better to factor out such constructs
to a common place (a new header) and hide them behind macros, which
can then be defined on a per-compiler basis.

This patch moves the existing definitions of function attributes and
likely/unlikely builtins to the new header. At the same time, it
unifies the names of these attribute defines and where they are
used. Moreover, it touches on jerry-main and removes the uses of
`__attribute__((unused))` entirely and replaces them with the
elsewhere used `(void) ...` pattern.

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

jerry-core/ecma/operations/ecma-get-put-value.c

@@ -50,7 +50,7 @@ ecma_op_get_value_lex_env_base (ecma_object_t *ref_base_lex_env_p, /**< referenc
   const bool is_unresolvable_reference = (ref_base_lex_env_p == NULL);
 
   /* 3. */
-  if (unlikely (is_unresolvable_reference))
+  if (JERRY_UNLIKELY (is_unresolvable_reference))
   {
 #ifdef JERRY_ENABLE_ERROR_MESSAGES
     ecma_value_t var_name_val = ecma_make_string_value (var_name_string_p);
@@ -151,7 +151,7 @@ ecma_op_put_value_lex_env_base (ecma_object_t *ref_base_lex_env_p, /**< referenc
   const bool is_unresolvable_reference = (ref_base_lex_env_p == NULL);
 
   /* 3. */
-  if (unlikely (is_unresolvable_reference))
+  if (JERRY_UNLIKELY (is_unresolvable_reference))
   {
     /* 3.a. */
     if (is_strict)