Optimize libc for generic armv7/armv8 neon cores

If (2nd) arch is "arm", (2nd) arch variant is "armv7-a-neon" or
"armv8-a", and (2nd) cpu variant is "generic", current Android.bp only
uses generic ARM assmebly or C code, even though they all have NEON.

This change use functions optimized for cortex-a15 (for these generic
cores), which are a reasonable balance for most ARM cores with Neon.

Also build script for specific armv7/armv8 cores are refactored to
take advantage of the common part (mostly cortex-a15 code), so more
than 150 repeated lines are removed.

Bug: 66064745
Test: For armv7-a-neon/armv8-a "generic" core
       - boot with GSI on sailfish
       - related cortex-a15 function are built and linked.
      All libc.a/libc.so built are identical for the following cores:
       - cortex-a7, cortex-a8, cortex-a9, cortex-a15, krait
       - cortex-a53, cortex-a53.a57, cortex-a73, denver, kryo
      $ lunch aosp_arm64 # with TARGET_2ND_CPU_VARIANT unset
      # For each $core listed above
      $ TARGET_2ND_CPU_VARIANT=$core m -e -j libc
Change-Id: Ia565d3ea09e034adcd014e7467a5b791cff908cb