Build uset targers as true PIE if user want to keep qemu
self-virtualizable.

v5:
  - Split into to patches: drop link hack and add PIE support
  - do not build PIE by default and drop toolchain check

v4:
  - Add test for toolchain if it has proper PIE support

v3:
  - One more pice of the hack was removed
  - Description updated

v2:
  - Add configure options do enable/disable PIE for usermode targets.
    Disabling can be useful if you build uswing toolchain which has
    broken PIE support. PIE for usermode targets enabled by default.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>