config: support clang for ARMv8 Linux

config: support clang for ARMv8 Linux

Moved all common defines from defconfig_arm64-armv8a-linuxapp-gcc
to common_armv8a_linuxapp.

Created new config arm64-armv8a-linuxapp-clang which adds the
clang support to armv8a.

Now defconfigs arm64-armv8a-linuxapp-gcc/clang contain only the
CONFIG_RTE_TOOLCHAIN* defines and all other common defines are
inherited from common_armv8a_linuxapp.

Signed-off-by: Ashwin Sekhar T K <ashwin.sekhar@caviumnetworks.com>
Reviewed-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
Acked-by: Jianbo Liu <jianbo.liu@linaro.org>
Acked-by: Hemant Agrawal <hemant.agrawal@nxp.com>