[LLDB] Add core definition for armv8l and armv7l

This patch adds core definitions in lldb ArchSpecs for armv8l and armv7l cores.

This was needed because on Linux running on 32-bit Arm v8 we are returned
armv8l in case we are running 32-bit sysroot on 64bit kernel. In case of 32-bit
kernel and 32-bit sysroot running on arm v8 hardware we are returned armv7l.
This is quite common when we run 32 bit arm using docker container.

Signed-off-by: Muhammad Omair Javaid <omair.javaid@linaro.org>

Differential Revision: https://reviews.llvm.org/D69904