PR middle-end/69780 - [4.9/5/6 Regression] ICE on __builtin_alloca_with_align

PR middle-end/69780 - [4.9/5/6 Regression] ICE on __builtin_alloca_with_align
	with small alignment
PR c/69759 - __builtin_alloca and __builtin_alloca_with_align undocumented

gcc/c-family/ChangeLog:
	* c-common.c (check_builtin_function_arguments): Validate and reject
	invalid arguments to __builtin_alloca_with_align.

gcc/ChangeLog:
	* doc/extend.texi (Other Builtins): Document __builtin_alloca and
	__builtin_alloca_with_align.

gcc/testsuite/ChangeLog:
	* g++.dg/ext/builtin_alloca.C: New test.
	* gcc.dg/builtins-68.c: New test.

From-SVN: r233640