[ATfE] Add multilib.yaml mapping form Armv8-A big endian to Armv7-A big endian (#234)

This is a cherry-pick of #230.

This mapping is necessary to make the multilib variant selection work
for Armv8-A big endian, since the library variants supposed to be used
in this case are chosen on the presence of the multilib flag
corresponding to Armv7-A.

Besides this change, this commit introduces more testing to cover more
cases of this mapping.

Author: vhscampos

arm-software/embedded/arm-multilib/multilib.yaml.in

@@ -158,9 +158,15 @@ Mappings:
 - Match: --target=(arm|thumb)v(8|8\.[1-9]|9|9\.[1-9])a-unknown-none-eabi
   Flags:
   - --target=armv7-unknown-none-eabi
+- Match: --target=(arm|thumb)ebv(8|8\.[1-9]|9|9\.[1-9])a-unknown-none-eabi
+  Flags:
+  - --target=armebv7-unknown-none-eabi
 - Match: --target=(arm|thumb)v(8|8\.[1-9]|9|9\.[1-9])a-unknown-none-eabihf
   Flags:
   - --target=armv7-unknown-none-eabihf
+- Match: --target=(arm|thumb)ebv(8|8\.[1-9]|9|9\.[1-9])a-unknown-none-eabihf
+  Flags:
+  - --target=armebv7-unknown-none-eabihf
 - Match: --target=(arm|thumb)v(8|8\.[1-9]|9|9\.[1-9])r-unknown-none-eabi
   Flags:
   - --target=armv7r-unknown-none-eabi