[RFC] Adding TLS selection choice to metadata

[Khem Raj <raj.khem@gmail.com>]

Hi

Enabling TLS support on uclibc/glibc is not sound enough in out build. Reason is we depend upon
various autotools tests to find out if the software combination we chose can support it or not
and these tests are not doing the correct think many times.

TLS is a libc feature from software POV. Implementation also depends upon hardware if there is
some special support available. 

Right now we have TLS support enabled by default in toolchain which works ok for newer glibc which have
TLS support in both Linuxthreads as well as nptl but it does not work so well with uclibc
and I think same will be case with klibc.

We need to disable it in the cross compiler while building the compiler so the TLS tests in 
subsequent applications which only tests for __thread acceptence dont assume that TLS support
is available in the system. 

I thought of putting it in metadata like TARGET_FPU and which can then be set from various other 
conf files.

A cheap workaround is that we could assume that TLS is only supported for glibc and disable it
for other libc variants but then NPTL is coming (whenever it comes) in uclibc so some architectures
in uclibc will support NPTL and some wont. Therefore defining it in metadata will give us the 
fine granularity to set it.

Ideas ?

Thanks

-Khem