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/parser/regexp/re-bytecode.c

@@ -123,7 +123,7 @@ re_bytecode_list_insert (re_bytecode_ctx_t *bc_ctx_p, /**< RegExp bytecode conte
  *
  * @return ecma character
  */
-inline ecma_char_t __attr_always_inline___
+inline ecma_char_t JERRY_ATTR_ALWAYS_INLINE
 re_get_char (uint8_t **bc_p) /**< pointer to bytecode start */
 {
   ecma_char_t chr;
@@ -137,7 +137,7 @@ re_get_char (uint8_t **bc_p) /**< pointer to bytecode start */
  *
  * @return current RegExp opcode
  */
-inline re_opcode_t __attr_always_inline___
+inline re_opcode_t JERRY_ATTR_ALWAYS_INLINE
 re_get_opcode (uint8_t **bc_p) /**< pointer to bytecode start */
 {
   uint8_t bytecode = **bc_p;
@@ -150,7 +150,7 @@ re_get_opcode (uint8_t **bc_p) /**< pointer to bytecode start */
  *
  * @return opcode parameter
  */
-inline uint32_t __attr_always_inline___
+inline uint32_t JERRY_ATTR_ALWAYS_INLINE
 re_get_value (uint8_t **bc_p) /**< pointer to bytecode start */
 {
   uint32_t value;
@@ -164,7 +164,7 @@ re_get_value (uint8_t **bc_p) /**< pointer to bytecode start */
  *
  * @return bytecode length (unsigned integer)
  */
-inline uint32_t __attr_pure___ __attr_always_inline___
+inline uint32_t JERRY_ATTR_PURE JERRY_ATTR_ALWAYS_INLINE
 re_get_bytecode_length (re_bytecode_ctx_t *bc_ctx_p) /**< RegExp bytecode context */
 {
   return ((uint32_t) (bc_ctx_p->current_p - bc_ctx_p->block_start_p));