Improve ld flags for CMake itself on Linux/HPPA

Workaround binutils bug by only marking unique section starting by
".text._".

This adds the HPPA workaround improved in commit b22a0f15 (bootstrap:
improve ld flag for Linux/HPPA builds, 2014-01-14) not only to the
bootstrap, but also to the normal build.
Merge topic 'hppa-binutils-flag' into next

8f2c2ccb Improve ld flags for CMake itself on Linux/HPPA