Add x86 / x86_arm mainline_system product

- Add mainline_x86 / mainline_x86_arm board
- Add mainline_system_x86 / mainline_system_x86_arm product
- BOARD_SYSTEM_EXTIMAGE_FILE_SYSTEM_TYPE := ext4

These products build only system.img and vbmeta.img.
These serve as a "golden example" for x86 / arm64 CSI.

Bug: 141972143
Bug: 141657175
Test: lunch mainline_system_x86-userdebug; m
Test: Compare $ANDROID_PRODUCT_OUT against sdk_phone_x86 / aosp_cf_x86_phone
Change-Id: I40dec69ceadfe7057c1706b883b81958d3c0a416