Fix master builds on mips* again (#815)

* Use local folder for xgo

* Always do crosscompile and testing to fail early

* Added mips* values for boltdb

In order to get master building again I have applied these 2 additional
files to boltdb. This should get dropped when
https://github.com/boltdb/bolt/issues/656 gets solved.

vendor/github.com/boltdb/bolt/bolt_mips64.go

@@ -0,0 +1,8 @@
+// +build mips64
+package bolt
+
+// maxMapSize represents the largest mmap size supported by Bolt.
+const maxMapSize = 0xFFFFFFFFFFFF // 256TB
+
+// maxAllocSize is the size used when creating array pointers.
+const maxAllocSize = 0x7FFFFFFF

vendor/github.com/boltdb/bolt/bolt_mips64le.go

@@ -0,0 +1,8 @@
+// +build mips64le
+package bolt
+
+// maxMapSize represents the largest mmap size supported by Bolt.
+const maxMapSize = 0xFFFFFFFFFFFF // 256TB
+
+// maxAllocSize is the size used when creating array pointers.
+const maxAllocSize = 0x7FFFFFFF