Add release notes for 12.6.0 release

Signed-off-by: Alin Jerpelea <alin.jerpelea@sony.com>

Signed-off-by: xuxin19 <xuxin19@xiaomi.com>

This patch attempts to fix regressions reported for pull# 12320.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

This fixes `tp` value of forked child in PROTECTED build to support
vfork. Why? the optimized `dispatch_syscall` requires `tp` to hold
the task TCB since commit e6973c7.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

This fixes stack pointer restore logic to avoid parent stack corruption
by forked child in PROTECTED build.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

This fixes PMP setting for open everything before going S-mode, which
has been broken since pull/12398.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>

Use `nxsched_gettid` instead of `nxsched_getpid`. Previously each
kernel thread was modelled as a task, so the pid is the same of the
kthread id. Now, with shared kthread group (introduced by #12320),
the pid of all kthreads will be 0 by design in shared group.

This allows using 0 base and size to depict the whole region.

Signed-off-by: Yanfeng Liu <yfliu2008@qq.com>