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/lit/lit-magic-strings.c

@@ -23,7 +23,7 @@
  * @return number of the strings, if there were registered,
  *         zero - otherwise.
  */
-inline uint32_t __attr_always_inline___
+inline uint32_t JERRY_ATTR_ALWAYS_INLINE
 lit_get_magic_string_ex_count (void)
 {
   return JERRY_CONTEXT (lit_magic_string_ex_count);