This patch changes logic around executing user signal handler
in kill() to make new signal handler thread use app TLS of an application
thread if available.

Normally, application threads share thread local storage area (TLS) with
kernel or use one created by kernel. But when running statically linked
executables or dynamically ones with Linux dynamic linker, the
application threads create TLS on its own and user signal handler
may reference thread local variables which do not exist in the TLS
of the new signal handler thread. As a result the app would crash
like so:

```
9  0x000000004030bd54 in page_fault (ef=0x40007fe83088) at arch/x64/mmu.cc:42
10 <signal handler called>
11 __GI___pthread_cleanup_upto (target=target@entry=0x200000205080 <sigend_jmp_buf>, targetframe=targetframe@entry=0x40007fe93fc8 "\005\"D\177") at ./nptl/pthread_cleanup_upto.c:32
12 0x000020007f44232c in _longjmp_unwind (env=env@entry=0x200000205080 <sigend_jmp_buf>, val=val@entry=1) at ../sysdeps/nptl/jmp-unwind.c:27
13 0x000020007f442205 in __libc_siglongjmp (env=0x200000205080 <sigend_jmp_buf>, val=1) at ../setjmp/longjmp.c:30
14 0x0000200000202543 in sigend_handler (sig=2) at main.c:121
15 0x000000004037f0ee in sched::thread::main (this=0x40007fe7e040) at core/sched.cc:1415
16 sched::thread_main_c (t=0x40007fe7e040) at arch/x64/arch-switch.hh:377
17 0x000000004030bc52 in thread_main () at arch/x64/entry.S:161
```

This patch applies a bit of trickery (potentially dangerous if user
handler tries to write to TLS) and select an application TLS of a
current thread or one of the other application threads and makes it
available to the new signal handler thread. The signal handler thread
in such case would switch from kernel TLS to app TLS before executing
handler routine.

This patch makes following tests pass when running with Linux
dynamic linker:
- tst-kill
- tst-sigwait
- tst-sigaction

Refers cloudius-systems#1278

Signed-off-by: Waldemar Kozaczuk <[email protected]>