signals: switch to app TLS if available

[wkozaczuk]

NOTE: This PR is a proposal and I am not sure we should merge it. Even though it fixes #1278 I have found that Golang apps signal handlers seem to validate that the stack used by signal handler routine was created by Golang. This means we may re-think how we handle user signal routine and possibly make it be executed on one of the application's threads (just like Linux does) instead of creating a new thread.

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 dynamic ones with Linux dynamic linker, the application threads create TLS on their own, and user signal handler may reference thread-local variables that 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 selects 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 #1278