toolchain-internal: skip gcc-intermediate when possible

toolchain-internal: skip gcc-intermediate when possible

When NPTL support was introduced, gcc required a three stages build
process. Since gcc 4.7, this is no longer necessary, and it is
possible to get back to a two stages build process. This patch takes
advantage of this, by doing a two stages build process when possible.

We introduce a few hidden kconfig options:

 * BR2_GCC_VERSION_NEEDS_THREE_STAGE_BUILD, which is set by the gcc
   Config.in logic to indicate that the compiler might need a three
   stages build. Currently, all versions prior to 4.7.x are selecting
   this kconfig option.

 * BR2_TOOLCHAIN_LIBC_NEEDS_THREE_STAGE_BUILD, which indicates whether
   the C library might need a three stages build. This is the case for
   eglibc, and uClibc when NPTL is enabled.

 * BR2_TOOLCHAIN_NEEDS_THREE_STAGE_BUILD finally is enabled when both
   of the previous options are enabled. It indicates that a three
   stages build is actually needed.

In addition to those options, the uClibc/gcc build logic is changed to
use only a two stages build process when possible.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>