Implement runtime::brk.

`brk` is used by some `malloc` implementations.

src/backend/linux_raw/runtime/syscalls.rs

@@ -263,3 +263,9 @@ unsafe fn sigtimedwait_old(
 pub(crate) fn exit_group(code: c::c_int) -> ! {
     unsafe { syscall_noreturn!(__NR_exit_group, c_int(code)) }
 }
+
+#[inline]
+pub(crate) unsafe fn brk(addr: *mut c::c_void) -> io::Result<()> {
+    // Don't mark this `readonly`, so that loads don't get reordered past it.
+    ret(syscall!(__NR_brk, addr))
+}

src/runtime.rs

@@ -510,3 +510,14 @@ pub unsafe fn sigtimedwait(set: &Sigset, timeout: Option<Timespec>) -> io::Resul
 pub fn linux_secure() -> bool {
     backend::param::auxv::linux_secure()
 }
+
+/// `brk(addr)`—Change the location of the “program break”.
+///
+/// # Safety
+///
+/// Be a good sport and don't break the allocator.
+#[cfg(linux_raw)]
+#[inline]
+pub unsafe fn brk(addr: *mut c_void) -> io::Result<()> {
+    backend::runtime::syscalls::brk(addr)
+}