libroot: Add brk() and sbrk().

This commit replaces the placeholder implementation of sbrk(), which
operated on a process' heap, with real implementations of brk() and
sbrk() that adjust a process' program break.

* unistd.h: Add standard definitions of brk() and sbrk(); include
stdint.h for intptr_t.
* thread.cpp: Recognize RLIMIT_AS and RLIMIT_DATA resource limits
(both currently unlimited); order limit identifiers alphabetically.
* arch-specific.cpp: Remove sbrk_hook().
* malloc_debug_api.cpp: Remove sbrk_hook().
* unistd/Jamfile: Build brk.c instead of sbrk.c.
* unistd/brk.c: Add.
* unistd/sbrk.c: Delete (placeholder implementation).
* libroot_stubs.c: Remove sbrk_hook().
* libroot_stubs_legacy.c: Remove sbrk_hook().
* src/tests/.../posix/Jamfile: Build brk_test.c.
* brk_test.c: Add (simple unit test that demonstrates behaviour of
sbrk()).

Signed-off-by: Jérôme Duval <jerome.duval@gmail.com>