testing: fix gcc/linker paths for several targets

There are also changes in the upstream docker images to support
more targets (android on ARM, etc).

Author: posborne

ci/cargo-config

@@ -16,10 +16,16 @@ linker = "gcc"
 linker = "arm-linux-androideabi-gcc"
 
 [target.arm-unknown-linux-gnueabihf]
-linker = "arm-linux-gnueabihf-gcc"
+linker = "arm-linux-gnueabihf-gcc-4.7"
 
 [target.mips-unknown-linux-gnu]
-linker = "mips-linux-gnu-gcc"
+linker = "mips-linux-gnu-gcc-5"
+
+[target.mipsel-unknown-linux-gnu]
+linker = "mipsel-linux-gnu-gcc-5"
 
 [target.aarch64-unknown-linux-gnu]
-linker = "aarch64-linux-gnu-gcc"
+linker = "aarch64-linux-gnu-gcc-4.8"
+
+[target.powerpc-unknown-linux-gnu]
+linker = "powerpc-linux-gnu-gcc-4.8"