tree-optimization/109609 - correctly interpret arg size in fnspec

tree-optimization/109609 - correctly interpret arg size in fnspec

By majority vote and a hint from the API name which is
arg_max_access_size_given_by_arg_p this interprets a memory access
size specified as given as other argument such as for strncpy
in the testcase which has "1cO313" as specifying the _maximum_
size read/written rather than the exact size.  There are two
uses interpreting it that way already and one differing.  The
following adjusts the differing and clarifies the documentation.

	PR tree-optimization/109609
	* attr-fnspec.h (arg_max_access_size_given_by_arg_p):
	Clarify semantics.
	* tree-ssa-alias.cc (check_fnspec): Correctly interpret
	the size given by arg_max_access_size_given_by_arg_p as
	maximum, not exact, size.

	* gcc.dg/torture/pr109609.c: New testcase.

(cherry picked from commit e8d00353017f895d03a9eabae3506fd126ce1a2d)