pixman: ignore NEON, IWMMXT, LOONGSON_MMI variables for class-native

pixman: ignore NEON, IWMMXT, LOONGSON_MMI variables for class-native

* pixman-native can have different do_configure sstate checksums if it's built with armv4t machine and armv7a
  OE @ ~ $ bitbake/bin/bitbake-diffsigs
    before-mgmt/stamps.1346795706/nokia900/x86_64-linux/pixman-native-*.do_configure.sigdata.*
    after-mgmt/stamps.1346801508/om-gta02/x86_64-linux/pixman-native-*.do_configure.sigdata.*
  basehash changed from 27e577de60880a788c7aaba797ef83e0 to c6799807eb3e767daf1e75738fc753f7
  Variable NEON value changed from   to  --disable-arm-neon
* so if you start building with different machine then last time (wrt
  NEON setting) all recipes which depends on pixman-native will be rebuilt too
* this explains why sstate-cache-management.sh wanted to remove many
  native sstate packages when --stamps-dir option was used (see comment
  28 in https://bugzilla.yoctoproject.org/show_bug.cgi?id=2897)

(From OE-Core rev: 0b466e6677208aeefdfa15aa37bd4681eda166c8)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>