toolchain-external: use BR2_GCC_TARGET_ARCH for toolchain conditionals

Instead of a long list of the ARMv7-A Cortex-A, use a conditional
based on BR2_GCC_TARGET_ARCH to hide/show toolchains that are only
usable on ARMv7-A.

However, in the comment related to Linaro toolchains, we keep
mentioning Cortex-A{5,8,9,15} because that's what users see when they
select their architecture variant.

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