valgrind: let the valgrind configure script detect TLS availability

valgrind: let the valgrind configure script detect TLS availability

Back in 2005, in commit
a2c326396a43ecbc8d02c3d815d4010a7ba2e004 ("update valgrind to the latest
and greatest"), an explicit --disable-tls option was added. More
recently, in commit 31a3f4bd54e12b8d6de286ab8fb6d9651990e2f5 ("valgrind:
enable tls support") changed this to be conditional on
BR2_GCC_ENABLE_TLS.

However, the configure script of valgrind is perfectly capable of
detecting TLS support, even in a cross-compilation case: it tries to
compile a program that uses __thread and sees if it works.

Since we're about to modify how BR2_GCC_ENABLE_TLS is handled, we'd
better remove its usage from packages, and valgrind is the only package
using this config option.

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