golang.org/x/sys/unix currently does not compile on mips nor mipsle as it fails to set the `isBigEndian` variable. This is due to mips and mipsle being missing in the build tag of their respective endian files.

This change adds mips to the build tag of `endian_big.go` and mipsle to the build tag of `endian_little.go` to fix this issue.

Change-Id: I79947cedb2645e80143e65bde00f2c95261125fb
Reviewed-on: https://go-review.googlesource.com/39730Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>