GIC held over 9 pct of Citigroup prior to stake sale * Now has below 5 pct; will stay on as “portfolio investor” * GIC says realised $1.6 bln profit from sale * Citigroup shares up in premarket trade By Kevin Lim and Saeed Azhar SINGAPORE, Sept 22 - Singapore wealth fund GIC has halved its stake in Citigroup <C.N>, cashing in on a market rally for a profit of $1.6 billion, but signalling investor concerns over the outlook for global banks. The sale follows other divestments by sovereign funds from Western banks they helped rescue at the start of the financial crisis to use the money closer to home in emerging markets and in other growth sectors such as resources. In June, Abu Dhabi exited Barclays Plc <BARC.L> with a gain of about 1.5 billion pounds , though Temasek [TEM.UL], another Singapore wealth fund, lost an estimated $4 billion from an early exit from Bank of ...