gold: Suppress "unused" variable warning on Clang

Clang generates a warning if there is a variable that is set but not used
otherwise ("-Wunused-but-set-variable").  On the default configuration, it
causes a build failure (unless "--disable-werror" is specified).

Because the cause of this error is in the Bison-generated code
($(srcdir)/gold/yyscript.y -> $(builddir)/gold/yyscript.c),
this commit suppresses this warning ("-Wunused-but-set-variable") by placing
DIAGNOSTIC_IGNORE_UNUSED_BUT_SET_VARIABLE macro at the end of user
prologue on yyscript.y.

	* yyscript.y: Suppress -Wunused-but-set-variable warning on
	the Bison-generated code.