BaseTools: Actually plug in BaseTools build on AArch64

Support for building BaseTools on AArch64 is available in the tree, but
not currently "plugged in". This patch adds the required snippet.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>



git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16169 6f19259b-4bc3-4df7-8a09-765794883524