Merge the memory allocation bits from libvex.h into main_util.c.
This is to avoid linkage problems due to unresolved symbols for
some compilers. See also valgrind r14600 and BZ #339542.


git-svn-id: svn://svn.valgrind.org/vex@2974 8f6e269a-dfd6-0310-a8e1-e2731360e62c
Bug 339858 arm64 recognize dmb sy. Data Memory Barrier full SYstem variant.

git-svn-id: svn://svn.valgrind.org/vex@2975 8f6e269a-dfd6-0310-a8e1-e2731360e62c