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-strings.c

@@ -557,7 +557,7 @@ lit_utf8_decr (const lit_utf8_byte_t **buf_p) /**< [in,out] buffer with characte
  *
  * @return ecma-string's hash
  */
-inline lit_string_hash_t __attr_always_inline___
+inline lit_string_hash_t JERRY_ATTR_ALWAYS_INLINE
 lit_utf8_string_hash_combine (lit_string_hash_t hash_basis, /**< hash to be combined with */
                               const lit_utf8_byte_t *utf8_buf_p, /**< characters buffer */
                               lit_utf8_size_t utf8_buf_size) /**< number of characters in the buffer */
@@ -580,7 +580,7 @@ lit_utf8_string_hash_combine (lit_string_hash_t hash_basis, /**< hash to be comb
  *
  * @return ecma-string's hash
  */
-inline lit_string_hash_t __attr_always_inline___
+inline lit_string_hash_t JERRY_ATTR_ALWAYS_INLINE
 lit_utf8_string_calc_hash (const lit_utf8_byte_t *utf8_buf_p, /**< characters buffer */
                            lit_utf8_size_t utf8_buf_size) /**< number of characters in the buffer */
 {
@@ -621,7 +621,7 @@ lit_utf8_string_code_unit_at (const lit_utf8_byte_t *utf8_buf_p, /**< utf-8 stri
  *
  * @return number of bytes occupied in CESU-8
  */
-inline lit_utf8_size_t __attr_always_inline___
+inline lit_utf8_size_t JERRY_ATTR_ALWAYS_INLINE
 lit_get_unicode_char_size_by_utf8_first_byte (const lit_utf8_byte_t first_byte) /**< buffer with characters */
 {
   if ((first_byte & LIT_UTF8_1_BYTE_MASK) == LIT_UTF8_1_BYTE_MARKER)