CcApiLibrary Variants for LLNDK

Define CcApiLibrary Variants for LLNDK. Each variant will be defined as
below.

cc_api_variant {
  name: "libc",
  variant: "llndk",
  arch: {
    x86_64: {
      src: "libs/x86_64/libc.so",
    },
    ...
  },
}

And CcApiLibrary will be marked to use this variant as below.

cc_api_library {
  name: "libc",
  ...
  variants: [
    "llndk",
    ...
  ]
}

Soong will replace source of libc cc_api_library as src from
cc_api_variant during build time.

Bug: 244244438
Test: cf vendor build succeeded with LLNDK variants
Change-Id: I317ed1f558beb1f82f9f2d70811fa8f765bcad2b