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

tests/unit-core/test-common.h

@@ -28,7 +28,7 @@
 #define TEST_ASSERT(x) \
   do \
   { \
-    if (unlikely (!(x))) \
+    if (JERRY_UNLIKELY (!(x))) \
     { \
       jerry_port_log (JERRY_LOG_LEVEL_ERROR, \
                       "TEST: Assertion '%s' failed at %s(%s):%lu.\n", \