toolchain: introduce a toolchain knob for NPTL

toolchain: introduce a toolchain knob for NPTL

As our architecture support expands to a number of architectures that
do not implement NPTL threading, and the number of packages that
depend on NPTL specific features, it has become necessary to be able
to know whether the toolchain has NPTL support or not.

This commit adds a new BR2_TOOLCHAIN_HAS_THREADS_NPTL hidden Config.in
option that allows packages to know whether NPTL is available or not.

This hidden option is:

 * Automatically enabled when glibc/eglibc or musl toolchains are
   used, either internal or external.

 * Automatically enabled when an internal uClibc toolchain with NPTL
   support is configured. It is left disabled otherwise for internal
   uClibc toolchains.

 * Configured according to a visible Config.in option for custom
   external uClibc toolchains.

[Peter: factor _EXTERNAL_HAS_THREADS in single if as suggested by Arnout]
Signed-off-by: Thomas Petazzoni <[email protected]>
Signed-off-by: Peter Korsgaard <[email protected]>