arch: some require a minimal gcc version

Hello All!

Not all architectures we support have support in upstream gcc, glibc, binutils, and/or linux. In some cases, only a few specific cores do not have that support.

Furthermore, some architecture, or specific cores, got support in a specific gcc version.

Currently, we have to account for this in at least three locations:
- the toolchain backend choice (internal or externsal)
- the gcc version choice
- each external toolchain

While a previous series handled the case for the internal backend, this second series caters to the minimal required gcc version.

New blind options are added, that archicetures or individual cores may select, to indicate they were introduced, or started to be supported in a specific gcc version.

The gcc version choice, as well as external toolchains, are then converted from ad-hoc (and sometimes incorrect) architecture dependencies, over to use these new required gcc version depednencies.

Note: the custom external toolchain is also converted, but this may cause a regression, in case a user has a custom toolchain with support for a specific arch or core back-ported. See the corresponding commit log for more details. In case this regression is deemed too important, we can simply drop the patch on the floor, in which case we could have the inverse situation, where a user uses a too old external toolchain in which the specific arch/core has not been back-ported, thus causing build failures.

Regards,
Yann E. MORIN.

The following changes since commit 0335a8a1164bfeb7c006c7d248d8f1923069c98f

arc/bfin: remove 60x cores (2017-09-03 11:09:53 +0200)

are available in the git repository at:

git://git.buildroot.org/~ymorin/git/buildroot.git

for you to fetch changes up to 41eeac74f986790fe26f03ebf05322120d2c04da

package/gcc: slight cleanup and reorg in remaining arch depends (2017-09-03 11:14:53 +0200)

----------------------------------------------------------------
Yann E. MORIN (8):
arch: introduce minimal required gcc version
package/gcc: hide versions too old for the current arch toolchain/external-custom: hide versions too old for the current arch toolchain/external: hide versions too old for the current arch arch/bfin: needs gcc >= 6
arch/mips: some variants need different gcc versions arch/arm: some variants need different gcc versions package/gcc: slight cleanup and reorg in remaining arch depends

 arch/Config.in                                     | 22 ++++++++++++++++
 arch/Config.in.arm                                 |  6 +++++
 arch/Config.in.mips                                |  7 ++++++
 package/gcc/Config.in.host                         | 29 +++++++---------------
 .../Config.in                                      |  1 +
 .../Config.in                                      |  1 +
 .../Config.in                                      |  1 +
 .../Config.in                                      |  1 +
 .../toolchain-external-codesourcery-arm/Config.in  |  6 ++---
 .../toolchain-external-codesourcery-mips/Config.in |  2 ++
 .../Config.in                                      |  1 +
 .../toolchain-external-custom/Config.in.options    | 15 +++++++++++
 .../toolchain-external-linaro-aarch64/Config.in    |  1 +
 .../toolchain-external-linaro-arm/Config.in        |  2 ++
 .../toolchain-external-linaro-armeb/Config.in      |  2 ++
 .../toolchain-external-synopsys-arc/Config.in      |  1 +
 16 files changed, 74 insertions(+), 24 deletions(-)

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