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/jcontext/jcontext.c

@@ -31,14 +31,14 @@ jerry_context_t jerry_global_context;
 #ifndef JERRY_HEAP_SECTION_ATTR
 #define JERRY_GLOBAL_HEAP_SECTION
 #else /* JERRY_HEAP_SECTION_ATTR */
-#define JERRY_GLOBAL_HEAP_SECTION __attribute__ ((section (JERRY_HEAP_SECTION_ATTR)))
+#define JERRY_GLOBAL_HEAP_SECTION JERRY_ATTR_SECTION (JERRY_HEAP_SECTION_ATTR)
 #endif /* !JERRY_HEAP_SECTION_ATTR */
 
 #ifndef JERRY_SYSTEM_ALLOCATOR
 /**
  * Global heap.
  */
-jmem_heap_t jerry_global_heap __attribute__ ((aligned (JMEM_ALIGNMENT))) JERRY_GLOBAL_HEAP_SECTION;
+jmem_heap_t jerry_global_heap JERRY_ATTR_ALIGNED (JMEM_ALIGNMENT) JERRY_GLOBAL_HEAP_SECTION;
 #endif /* !JERRY_SYSTEM_ALLOCATOR */
 
 #ifndef CONFIG_ECMA_LCACHE_DISABLE