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/base/ecma-helpers-conversion.c

@@ -818,7 +818,7 @@ ecma_uint32_to_utf8_string (uint32_t value, /**< value to convert */
 
   lit_utf8_size_t bytes_copied = (lit_utf8_size_t) (out_buffer_p + buffer_size - buf_p);
 
-  if (likely (buf_p != out_buffer_p))
+  if (JERRY_LIKELY (buf_p != out_buffer_p))
   {
     memmove (out_buffer_p, buf_p, bytes_copied);
   }
@@ -960,7 +960,7 @@ ecma_number_to_decimal (ecma_number_t num, /**< ecma-number */
  *
  * @return number of digits
  */
-inline static int32_t __attr_always_inline___
+inline static int32_t JERRY_ATTR_ALWAYS_INLINE
 ecma_number_of_digits (double val) /**< ecma number */
 {
   JERRY_ASSERT (fabs (fmod (val, 1.0)) < EPSILON);
@@ -978,7 +978,7 @@ ecma_number_of_digits (double val) /**< ecma number */
 /**
  * Convert double value to ASCII
  */
-inline static void __attr_always_inline___
+inline static void JERRY_ATTR_ALWAYS_INLINE
 ecma_double_to_ascii (double val, /**< ecma number */
                       lit_utf8_byte_t *buffer_p, /**< buffer to generate digits into */
                       int32_t num_of_digits, /**< number of digits */
@@ -1012,7 +1012,7 @@ ecma_double_to_ascii (double val, /**< ecma number */
  *
  * @return number of generated digits
  */
-static inline lit_utf8_size_t __attr_always_inline___
+static inline lit_utf8_size_t JERRY_ATTR_ALWAYS_INLINE
 ecma_double_to_binary_floating_point (double val, /**< ecma number */
                                       lit_utf8_byte_t *buffer_p, /**< buffer to generate digits into */
                                       int32_t *exp_p) /**< [out] exponent */