x/build: add a "small" builder with limited resources

[mvdan]

Starting a new issue from #27739.

We tend to see issues building Go and running the tests if someone's machine has limited resources. For example, in #26867 I reported how go test net OOM'd with a few gigabytes of available memory.

We already have special builders like linux-amd64-noopt, so I propose adding a linux-amd64-small. Alternatively, we could add these qualities to an existing builder like linux-amd64-noopt, like @bradfitz suggested.

Some ideas to start with:

• limiting the total memory to 4GB
• limiting the total disk size to 10GB
• limiting the CPU power, e.g. to dual-core 1GHz

With time, if the builder is stable, we could lower those numbers and add more restrictions, such as:

• lowering the maximum number of open file descriptors
• lowering the size of /tmp
• lowering the maximum number of processes created by the user

/cc @dmitshur @bradfitz @andybons