test-release.sh: Add a way to specify --build to configure

Summary:
I currently have to specify --build=mips-linux-gnu or --build=mipsel-linux-gnu
to configure in order to successfully recurse a 32-bit build of the compiler on
my mips64-linux-gnu and mips64el-linux-gnu targets. This is a bug and will be
fixed but in the meantime it will be useful to have a way to work around this.

Reviewers: tstellarAMD

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6522