gnu: gcc: Do not pass -dynamic-linker to linker when !shared on arm.

Fixes <http://bugs.gnu.org/20102

>.

* gnu/packages/patches/gcc-arm-link-spec-fix.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/gcc.scm (gcc-4.8, gcc-4.9): Add patch.
* gnu/packages/cross-base.scm (cross-gcc): Preserve patches from gcc-4.8.

Co-Authored-By: Ludovic Courtès <ludo@gnu.org>