Android's elf.h doesn't have PT_GNU_STACK (sigh) and so r13368 broke
the build.  This conditionalises that fragment on the present of
PT_GNU_STACK.


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13375 a5019735-40e9-0310-863c-91ae7b9d1cf9