Add doc for branch_cost effective target.

2018-01-12  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* doc/sourcebuild.texi (Effective-Target Keywords, Other attributes):
	Add item for branch_cost.

From-SVN: r256574

gcc/ChangeLog

+2018-01-12  Christophe Lyon  <christophe.lyon@linaro.org>
+
+	* doc/sourcebuild.texi (Effective-Target Keywords, Other attributes):
+	Add item for branch_cost.
+
 2018-01-12  Eric Botcazou  <ebotcazou@adacore.com>
 
 	PR rtl-optimization/83565

gcc/doc/sourcebuild.texi

@@ -2133,6 +2133,9 @@ Target supports wide characters.
 @item automatic_stack_alignment
 Target supports automatic stack alignment.
 
+@item branch_cost
+Target supports @option{-branch-cost=N}.
+
 @item cxa_atexit
 Target uses @code{__cxa_atexit}.