Add armv8-a as a valid/supported 32-bit arm architecture variant. If
some known armv8-a core, like cortex-a53, cortex-a73, etc. is specified
as TARGET(_2ND)_CPU_VARIANT, the associated TARGET(_2ND)_ARCH_VARIANT
will be treated as if armv8-a is specified.
Bug: 62895439
Test: "bionic-unit-tests-static --gtest_filter=*strlen*" on Nexus 4
(armv7-krait), emulator (armv7), and sailfish (armv8-kryo).
The test binary for the first 2 is built with armv7-a as its
TARGET_ARCH_VARIANT; The test binary for the last is built with
armv8-a as its TARGET_2ND_ARCH_VARIANT.
TARGET(_2ND)_CPU_VARIANTs of both binaries are set to "generic".
Change-Id: I53bee2974346cf485b2da54cf7aad010b0425910
9c02066b13