Android: Unknown processor CMAKE_SYSTEM_PROCESSOR=arm

[MarginGitHub]

When this happens，CMAKE_SYSTEM_PROCESSOR should be armv7-a，not arm

[adwhit]

For anyone arriving here after trying to upgrade to NDK v27, I think there is now a separate issue that has surfaced that gives a similar error. The root cause is actually the ABI values not being set at all within cmake due to a change in configuration. See: android/ndk#2049