arch/Config.in: introduce BR2_ARCH_NEEDS_GCC_AT_LEAST_16

gcc 15 was introduced in Buildroot in commit [1].

In Buildroot, a user creates a build configuration in Kconfig
menus by selecting the CPU architecture and CPU first, then
a toolchain that support this CPU, then packages and system
configuration.

To make sure those Kconfig menus and choices behave correctly in that
order, we use a logic in which a CPU selects the Kconfig symbol
BR2_ARCH_NEEDS_GCC_AT_LEAST_N (with N being the needed gcc version).
Then, to make sure unsupported gcc version are hidden for a selected
CPU, a gcc version N has in fact a "depends on
!BR2_ARCH_NEEDS_GCC_AT_LEAST_(N+1)". For example, Cortex-A710 needs
at least gcc 12. See [2] and [3].

This means that when we introduce the latest gcc version with this
logic in place, we already need this BR2_ARCH_NEEDS_GCC_AT_LEAST_(N+1)
symbol defined. Doing so is desirable to avoid forgetting to add it
later. See for example commit [4], adding those missing dependencies
for gcc 13 and 14.

Since in Buildroot we currently keep 3 actively maintained gcc
versions, adding a new gcc version N in Buildroot should:
- add the BR2_ARCH_NEEDS_GCC_AT_LEAST_(N+1) symbol
- add package/gcc N
- switch the default gcc from N-2 to N-1
- drop package/gcc N-3

Then, once this new gcc N is added, new CPUs supported by that version
can be introduced, by selecting BR2_ARCH_NEEDS_GCC_AT_LEAST_N.

All this logic will be ready for future gcc versions. This was also
described in commit log [5].

This commit simply adds the BR2_ARCH_NEEDS_GCC_AT_LEAST_16 symbol.

It is worth mentioning that, at the time of this commit, gcc 16 is not
released yet. This release number 16 is already known, though. See [6].

For reference, this was discussed in [7].

[1] 75891397ab
[2] https://gitlab.com/buildroot.org/buildroot/-/blob/2025.05/arch/Config.in.arm#L514
[3] https://gitlab.com/buildroot.org/buildroot/-/blob/2025.05/package/gcc/Config.in.host#L27
[4] 6f231d3003
[5] eab17d763c
[6] https://gcc.gnu.org/gcc-16/changes.html
[7] https://lore.kernel.org/buildroot/877c0v1sn1.fsf@dell.be.48ers.dk/T/#m45688e25632401029e4c0d989072e1d3e1a59854

Signed-off-by: Julien Olivain <ju.o@free.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

arch/Config.in

@@ -311,6 +311,10 @@ config BR2_ARCH_NEEDS_GCC_AT_LEAST_15
 	bool
 	select BR2_ARCH_NEEDS_GCC_AT_LEAST_14
 
+config BR2_ARCH_NEEDS_GCC_AT_LEAST_16
+	bool
+	select BR2_ARCH_NEEDS_GCC_AT_LEAST_15
+
 # The following string values are defined by the individual
 # Config.in.$ARCH files
 config BR2_ARCH