x/build: create mips32 soft-float builder

[aclements]

Commit 6be1c09 finished support for mips32 soft-float. We should have a builder configuration to test this.

[bradfitz]

/cc @dmitshur

[dmitshur]

@bradfitz From x/build/dashboard/builders.go, it looks like all our current mips builders are reverse builders run by @MIPSbkirby, is that right?

Does that mean we'll either need to create the first direct builder of this type (and the accompanying environment for it) or ask @MIPSbkirby if he'd be willing to add this new configuration?

Are we interested in adding a full builder, or just a misc-compile one (compiles but doesn't run tests)? We do have a misc-compile-mips builder ourselves.

[bradfitz]

We don't need the cooperation of @MIPSbkirby.

We can reuse the host type and only add a new builder type with a different environment. And we can also restrict which tests it runs and just do some representative ones. (runtime, math, maybe time?)