toolchain-external-linaro-{arm,armeb}: allow on ARMv8

toolchain-external-linaro-{arm,armeb}: allow on ARMv8

The Linaro toolchains are currently only available on ARMv7-A, but can
in fact also be used to generate 32 bits code for ARMv8 platforms. This
commit therefore adjusts their architecture dependency.

Example, a 32 bits ARM build produces a 32 bits busybox binary:

$ file output/target/bin/busybox
output/target/bin/busybox: setuid ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 2.6.32, BuildID[sha1]=16a7a70eb9cac08759e52a260478b9c287f59238, stripped

Which was built for Cortex-A72:

$ ./output/host/usr/bin/arm-linux-gnueabihf-readelf -A output/target/bin/busybox
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "Cortex-A72"
  Tag_CPU_arch: v8
  Tag_CPU_arch_profile: Application
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2
  Tag_FP_arch: FP for ARMv8
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_rounding: Needed
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_align_preserved: 8-byte, except leaf SP
  Tag_ABI_enum_size: int
  Tag_ABI_VFP_args: VFP registers
  Tag_CPU_unaligned_access: v6
  Tag_MPextension_use: Allowed
  Tag_Virtualization_use: TrustZone and Virtualization Extensions

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