x/mobile: needs to build standard library as PIC for android

[mwhudson]

gomobile installs the standard library for you but it just compiles it with the default flags. Thanks to the use of -pkgdir, the attempts made by the go tool to build the runtime as PIC when -buildmode=c-shared is passed fail.

[crawshaw]

I'll take a look at this next week. We probably need to detect whether we are working with Go 1.5 or tip.

[gopherbot]

CL https://golang.org/cl/15803 mentions this issue.