README - alpha switched to GCC 5.3

[matthew green <mrg%eterna.com.au@localhost>]

hi folks.


i've switched the alpha port to GCC 5.3.  everything seems to work fine
for me.  there is one new (common to all GCC 5.3 ports) ATF failure in
expr, and several new working tests.

please send-pr any issues you see! thanks.


.mrg.