go: Update usage of TARGET_AIX to TARGET_AIX_OS

TARGET_AIX is defined to a non-zero value on linux and maybe other
powerpc64le targets.  This leads to unexpected behavior such as
dropping the .go_export section when linking a shared library
on linux/powerpc64le.

Instead, use TARGET_AIX_OS to toggle AIX specific behavior.

Fixes golang/go#60798.

2023-06-22  Paul E. Murphy  <murphyp@linux.ibm.com>

gcc/go/
	* go-backend.cc [TARGET_AIX]: Rename and update usage to TARGET_AIX_OS.
	* go-lang.cc: Likewise.

(cherry picked from commit b76cd1ec)