uclibc: adapt thread implementation selection to uClibc-ng

uClibc-ng does not support linuxthreads or linuxthreads.old on
architectures that have NPTL support. This creates another complicated
dependency: dependeing on the uClibc version being used, not the same
thread implementations are available.

In order to handle this situation, this patch introduces three hidden
booleans:

 - BR2_UCLIBC_VERSION_SUPPORTS_LINUXTHREADS
 - BR2_UCLIBC_VERSION_SUPPORTS_LINUXTHREADS_OLD
 - BR2_UCLIBC_VERSION_SUPPORTS_NPTL

They are selected by the different uClibc versions, depending on which
thread implementation they support on the different architectures.

Then, the choice of the thread implementation can rely on those
booleans to know if a given thread implementation is available in the
current architecture / uClibc version selection.

This makes sure that unusable thread implementation do not get
selected, therefore fixing build issues such as:

  http://autobuild.buildroot.org/results/89e/89e423bee040cbce3e82cd89f1191efaac490c0d/



The support table is as follows (only taking into account
architectures that allow the selection of
BR2_TOOLCHAIN_BUILDROOT_UCLIBC, other architectures are not
considered) :

             ----uclibc----   uclibc-xtensa-   --uclibc-arc--  --uclibc-ng---
             LT LT.old NPTL   LT LT.old NPTL   LT LT.old NPTL  LT LT.old NPTL
arc(le|eb)                                      y   y     n    n    n     n (1)
arm(eb)      y    y     y                                      n    y     y (2)
bfin         n    y     n                                      y    y     n
i386         y    y     y                                      n    n     y (3)
m68k         y    y     y                                      y    y     n
mips(64)(el) y    y     y                                      n    n     y
powerpc      y    y     y                                      n    n     y
sh           y    y     y                                      n    n     y
sparc        y    y     y                                      n    n     y
xtensa                        n    y     n                     n    n     y
x86_64       y    y     y                                      n    n     y

  (1) : uclibc-ng only has NPTL support for ARC but it requires a more
        recent compiler version that hasn't been officially released
        by Synopsys.

  (2) : the general idea of uclibc-ng is to only support NPTL on
        architectures where it is available. However, in order to
        support ARM noMMU platforms, LT.old support has been kept on
        ARM.

  (3) : except i386 itself, which doesn't have what's needed for NPTL
        support. i386 is simply not supported by uclibc-ng basically.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>