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-errol.c

@@ -73,7 +73,7 @@ typedef struct
 /**
  * Normalize the number by factoring in the error.
  */
-static inline void __attr_always_inline___
+static inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_normalize_high_prec_data (ecma_high_prec_t *hp_data_p) /**< [in, out] float pair */
 {
   double val = hp_data_p->value;
@@ -85,7 +85,7 @@ ecma_normalize_high_prec_data (ecma_high_prec_t *hp_data_p) /**< [in, out] float
 /**
  * Multiply the high-precision number by ten.
  */
-static inline void __attr_always_inline___
+static inline void JERRY_ATTR_ALWAYS_INLINE
 ecma_multiply_high_prec_by_10 (ecma_high_prec_t *hp_data_p) /**< [in, out] high-precision number */
 {
   double value = hp_data_p->value;
@@ -127,7 +127,7 @@ ecma_divide_high_prec_by_10 (ecma_high_prec_t *hp_data_p) /**< [in, out] high-pr
  *
  * @return number of generated digits
  */
-inline lit_utf8_size_t __attr_always_inline___
+inline lit_utf8_size_t JERRY_ATTR_ALWAYS_INLINE
 ecma_errol0_dtoa (double val, /**< ecma number */
                   lit_utf8_byte_t *buffer_p, /**< buffer to generate digits into */
                   int32_t *exp_p) /**< [out] exponent */