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.h

@@ -103,7 +103,7 @@ typedef struct
 re_opcode_t re_get_opcode (uint8_t **bc_p);
 ecma_char_t re_get_char (uint8_t **bc_p);
 uint32_t re_get_value (uint8_t **bc_p);
-uint32_t re_get_bytecode_length (re_bytecode_ctx_t *bc_ctx_p) __attr_pure___;
+uint32_t JERRY_ATTR_PURE re_get_bytecode_length (re_bytecode_ctx_t *bc_ctx_p);
 
 void re_append_opcode (re_bytecode_ctx_t *bc_ctx_p, re_opcode_t opcode);
 void re_append_u32 (re_bytecode_ctx_t *bc_ctx_p, uint32_t value);