string: Compile memcpy-sve.S for aarch64 if compiler supports it

This is a partial revert of b7e368fb. If SVE assembly is guarded by
__ARM_FEATURE_SVE, it cannot build when SVE is not enabled by the build
system. This is ok on AOR, but because Android (bionic) uses ifuncs to
select the appropriate assembly at runtime, these need to compile
regardless of if the target actually supports the instructions.

Check for AArch64 and GCC >= 8 or Clang >= 5 so that SVE is not used on
compilers that do not support it. This condition will always be true on
future builds of Android for AArch64.