toolchain-external: use BR2_ARM_CPU_ARM* options

Refactor the toolchain-external Config.in file to use the
BR2_ARM_CPU_ARM*. All of the changes are purely mechanical, except for
the Arago ARMv5 toolchain: it had a 'depends on BR2_GCC_TARGET_ARCH !=
"armv5t"', but armv5t was not a possible value for
BR2_GCC_TARGET_ARCH. Since the toolchain is ARMv5TE, the only ARM
architectures we need to exclude are ARMv4 and ARMv4T.

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