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/js/js-parser-statm.c

@@ -701,7 +701,7 @@ parser_parse_while_statement_start (parser_context_t *context_p) /**< context */
 /**
  * Parse while statement (ending part).
  */
-static void __attr_noinline___
+static void JERRY_ATTR_NOINLINE
 parser_parse_while_statement_end (parser_context_t *context_p) /**< context */
 {
   parser_while_statement_t while_statement;
@@ -947,7 +947,7 @@ parser_parse_for_statement_start (parser_context_t *context_p) /**< context */
 /**
  * Parse for statement (ending part).
  */
-static void __attr_noinline___
+static void JERRY_ATTR_NOINLINE
 parser_parse_for_statement_end (parser_context_t *context_p) /**< context */
 {
   parser_for_statement_t for_statement;
@@ -1028,7 +1028,7 @@ parser_parse_for_statement_end (parser_context_t *context_p) /**< context */
 /**
  * Parse switch statement (starting part).
  */
-static void __attr_noinline___
+static void JERRY_ATTR_NOINLINE
 parser_parse_switch_statement_start (parser_context_t *context_p) /**< context */
 {
   parser_switch_statement_t switch_statement;
@@ -2202,7 +2202,7 @@ parser_parse_statements (parser_context_t *context_p) /**< context */
 /**
  * Free jumps stored on the stack if a parse error is occured.
  */
-void __attr_noinline___
+void JERRY_ATTR_NOINLINE
 parser_free_jumps (parser_stack_iterator_t iterator) /**< iterator position */
 {
   while (true)