RFR(xs): 8152356: Change to -Xshare:auto for 32-bit platforms

[Calvin Cheung]

Please review this small fix for enabling -Xshare:auto as the default 
for 32-bit platforms.

JBS: https://bugs.openjdk.java.net/browse/JDK-8152356

webrev: http://cr.openjdk.java.net/~ccheung/8152356/webrev.00/

Testing:
     JPRT
     jtreg tests under hotspot/test/runtime

thanks,
Calvin