middle-end/106331 - fix mem attributes for string op arguments

middle-end/106331 - fix mem attributes for string op arguments

get_memory_rtx tries hard to come up with a MEM_EXPR to record
in the memory attributes but in the last fallback fails to properly
account for an unknown offset and thus, as visible in this testcase,
incorrect alignment computed from set_mem_attributes.  The following
rectifies both parts.

	PR middle-end/106331
	* builtins.cc (get_memory_rtx): Compute alignment from
	the original address and set MEM_OFFSET to unknown when
	we create a MEM_EXPR from the base object of the address.

	* gfortran.dg/pr106331.f90: New testcase.

(cherry picked from commit e4ff11a8f2e80adb8ada69bf35ee6a1ab18a9c85)