Initial data (brk) segment is setup on demand, when a first brk() syscall

Initial data (brk) segment is setup on demand, when a first brk() syscall
is made. It cannot be established during client image initialization because
that would conflict with a temporary stack which ld.so.1 (when executed directly)
uses for loading the target dynamic executable.
See PRE(sys_brk) in syswrap-solaris.c.

Preparatory work for ldsoexec support.
n-i-bz

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15572