Build fails w/ GOOS of "mips" or "mipsle"

[josephlr]

Running either of the following commands:

GOARCH=mips go build github.com/google/fscrypt/...
GOARCH=mipsle go build github.com/google/fscrypt/...

results in the following error:

# github.com/google/fscrypt/util
util/util.go:47:11: type [2147483647]byte larger than address space

This has broken the Debian package for mips and mipsle.

This failure comes from my error here:

// Silce must fix in 32-bit address space to build on 32-bit platforms.
return (*[math.MaxInt32]byte)(ptr)[:]

The docs clearly say that 1 << 30 should be used instead of math.MaxInt32. There have been discussions here and here about this, but they haven't ever resulted in anything nicer to use.

[josephlr]

We should also fix our CI to at least make sure that fscrypt builds on all supported GOOS values