Stash TARGET_ARCH_VARIANT_CPU and print it in dumpvar.

Bug:7961327

Merged from internal master.

Change-Id: Ia78472ba53ae1221de4572119aa5c63246941eeb
build/core: Dont override arch_variant_ldflags

Change-Id: I95a4a18594c6468ea8660f935296a99a544fa2d0