Add configure option to compile user targets as PIE

Add configure option to compile user targets as PIE

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>