linux-user: Fix brk() to release pages

linux-user: Fix brk() to release pages

The current brk() implementation does not de-allocate pages if a lower
address is given compared to earlier brk() calls.
But according to the manpage, brk() shall deallocate memory in this case
and currently it breaks a real-world application, specifically building
the debian gcl package in qemu-user.

Fix this issue by reworking the qemu brk() implementation.

Tested with the C-code testcase included in qemu commit 4d1de87c750, and
by building debian package of gcl in a hppa-linux guest on a x86-64
host.

Signed-off-by: Helge Deller <deller@gmx.de>
Message-Id: <Y6gId80ek49TK1xB@p100>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>