rust: Update usage of TARGET_AIX to TARGET_AIX_OS

This was noticed when fixing the gccgo usage of the macro, the
rust usage is very similar.

TARGET_AIX is defined as a non-zero value on linux/powerpc64le
which may cause unexpected behavior.  TARGET_AIX_OS should be
used to toggle AIX specific behavior.

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

gcc/rust/
	* rust-object-export.cc [TARGET_AIX]: Rename and update usage to
	TARGET_AIX_OS.