Correct handling of the TLS/NetBSD block of the main program

Summary:
Include <sys/tls.h> for:

 - struct tls_tcb - thread control block structure
 - __HAVE___LWP_GETTCB_FAST - __lwp_gettcb_fast() is available
 - __HAVE___LWP_GETPRIVATE_FAST -  __lwp_getprivate_fast() is available
 - __HAVE_TLS_VARIANT_I - TLS Variant I for this architecture
 - __HAVE_TLS_VARIANT_II - TLS Variant II for this architecture

Rename ThreadSelfSegbase() to ThreadSelfTlsTcb and switch it
to retrieve in a portable way TCB.

Switch ThreadSelf() to retrieve pthread from struct tcb_tls.

Use dl_iterate_phdr() to find out the size of TLS block of
the main program.

Correct the index of the  TLS block of the main program
(dlpi_tls_modid); it's 1, not 2.

New NetBSD code is now CPU (NetBSD port) agnostic.

Stop sharing the same code with FreeBSD.

Sponsored by <The NetBSD Foundation>

Reviewers: dvyukov, joerg, vitalybuka

Reviewed By: vitalybuka

Subscribers: emaste, kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40159

llvm-svn: 320234