RFR(XS): 8220690: ATTRIBUTE_ALIGNED requires GNU extensions enabled

[Kim Barrett]

Please review this change to the GCC-version of the ATTRIBUTE_ALIGNED
macro, to not require enabling GNU extensions.  Instead of using a
typeof form we use __typeof__ instead.  See the CR for more details.

CR:
https://bugs.openjdk.java.net/browse/JDK-8220690

Webrev:
http://cr.openjdk.java.net/~kbarrett/8220690/open.00/

Testing:
mach5 tier1