cmd/link: do not pass -no-pie to host linker when -linkshared is passed

As the comment above the code I'm changing says, when building with
-buildmode=exe, the default compiler flags produce code incompatible with PIE.
But when -linkshared is passed, the default compiler flags are not used so this
does not apply. And now I've found a system (linux/arm64 with glibc 2.27) where
this combination of flags causes a problem, albeit for reasons I don't really
understand, so stop passing -no-pie when -linkshared is passed.

Change-Id: I412ec7941dc0cb89e6d1b171fc29288aadcb9f20
Reviewed-on: https://go-review.googlesource.com/104815


Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>