gcc: remove BR2_GCC_ENABLE_TLS option

gcc: remove BR2_GCC_ENABLE_TLS option

The current BR2_GCC_ENABLE_TLS can cause users to make incorrect
choices, and is not very useful. This options allows to decide whether
we pass --enable-tls or --disable-tls to gcc, to enable or disable
support for Thread Local Storage.

Its behavior is:

 - The option is default to "y" but only exists if we're using
   uClibc/NPTL or glibc.

 - When we're using uClibc, the option can be disabled.

So, in practice, this means that currently:

 - TLS support is always on for glibc

 - TLS support is on by default for uClibc/NPTL, but can be disabled in
   the configuration. This is in fact bad and causes the build failure
   reported in bug #7424 (this bug is still reproducible on master)

 - TLS support is always disabled for uClibc/no-thread and
   uClibc/linuxthreads.

 - TLS support is always disabled for musl. This does not cause any
   build failure, but musl can use TLS support, and therefore be more
   efficient. According to
   http://www.openwall.com/lists/musl/2012/10/04/1, "Note that if you've
   been building gcc with --disable-tls, __thread was already working
   but gets emulated (very poorly; it's slow and will abort() if it runs
   out of memory) through libgcc.".

So, this commit completely removes the BR2_GCC_ENABLE_TLS and instead
makes the right choice inside gcc.mk directly:

 - TLS support enabled for glibc, musl and uClibc/NPTL

 - TLS support in other cases, i.e uClibc/no-thread and
   uClibc/linuxthreads.

We have intentionally *not* added the option to
Config.in.legacy. Indeed, the new behavior is *exactly* the same as the
older behavior, with the exception of:

 - People can no longer disable TLS support in uClibc/NPTL, which was
   anyway causing a build failure and therefore was not used.

 - TLS support is now enabled on musl, but people using musl already had
   BR2_GCC_ENABLE_TLS not set, so they wouldn't get the legacy warning.

Fixes bug #7424.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>