issue-518: add support for x32 ABI to linux_syscall_support.h

As pointed out in the ticket this is taken from chromium review system
here: https://codereview.chromium.org/13648012



git-svn-id: http://gperftools.googlecode.com/svn/trunk@218 6b5cf1ce-ec42-a296-1ba9-69fdba395a50