Remove redundant extern keywords from function declarations/definitions. (#1495)

Extern keywords on function declarations/definitions provide no additional value since function declarations/definitions default to external linkage in C99, e.g. removing them won't change the semantics of the program.

The extern keywords were essentially a legacy from the early days of the project. This commit cleans this up across the whole codebase in one go to minimize history disruption.

The bulk of the changes in this commit were produced by a custom clang-tidy checker.

Note that variables declarations carrying the extern keyword are untouched by this commit since there the presence of the keyword actually has an impact on the semantics of the program.

JerryScript-DCO-1.0-Signed-off-by: Tilmann Scheller t.scheller@samsung.com

jerry-core/ecma/operations/ecma-arraybuffer-object.h

@@ -25,17 +25,17 @@
  * @{
  */
 
-extern ecma_value_t
+ecma_value_t
 ecma_op_create_arraybuffer_object (const ecma_value_t *, ecma_length_t);
 
 /**
  * Helper functions for arraybuffer.
  */
-extern ecma_object_t *
+ecma_object_t *
 ecma_arraybuffer_new_object (ecma_length_t);
-extern lit_utf8_byte_t *
+lit_utf8_byte_t *
 ecma_arraybuffer_get_buffer (ecma_object_t *) __attr_pure___;
-extern ecma_length_t
+ecma_length_t
 ecma_arraybuffer_get_length (ecma_object_t *) __attr_pure___;
 
 /**