multilib-build: add flags for arm/arm64

Add abi_arm_32 and abi_arm_64 which we require for ARM64 ARC++.

BUG=b:139030617
TEST=Build kevin-arc64

Cq-Depend: chromium:1746929
Change-Id: I1622ed740e80179a89f9a182d655ac17987bc5f6
Reviewed-on: https://chromium-review.googlesource.com/1753243
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: David Jacobo <djacobo@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>

eclass/multilib-build.eclass

diff --git a/eclass/multilib-build.eclass b/eclass/multilib-build.eclass
index 05917ba..3df63aa 100644
--- a/eclass/multilib-build.eclass
+++ b/eclass/multilib-build.eclass
@@ -49,6 +49,8 @@
 	abi_ppc_64:ppc64
 	abi_s390_32:s390
 	abi_s390_64:s390x
+	abi_arm_32:arm
+	abi_arm_64:arm64
 )
 readonly _MULTILIB_FLAGS