Unify expected strings for arm architecture

[ghost]

In the documentation as well as in other parts of build.sh "armv7" and "armv8" are used to decide the build architecture.

Currently there's no problem if build_apprtc() is used because it overwrites WEBRTC_ARCH. But if you only want to build one architecture and follow the readme, it says that

export WEBRTC_ARCH=armv7 or export WEBRTC_ARCH=armv8

should be used. In that case, the if inside execute_build() will fail because is looking for "armeabi-v7a" and "arm64-v8a"

[ghost]

The other line changes are because my text editor removes trailing whitespaces

[ArikYa]

Thank you @lucaslafarga fir these updates