bfin: fix issues with internal toolchain, re-enable C++ support

bfin: fix issues with internal toolchain, re-enable C++ support

The three patches allow to compile applications using TLS emulation from
libgcc or C++ applications.

The patches 892-libgcc-mkmap-symver-support-skip_underscore.patch and
893-libgcc-config-bfin-use-the-generic-linker-version-in.patch fixes how
libgcc is generated, by making the necessary libgcc symbols declared
"GLOBAL", and therefore visible outside of libgcc. This fixes a large
number of undefined reference issues (for either C++ applications or
applications using TLS emulation). This was reported as gcc PR74748.

The patch 894-libgcc-fix-DWARF-compilation-with-FDPIC-targets.patch
allows to build DWARF in FDPIC mode. This patch replaces the older
892-disable-dwarf-bfin.patch, as instead of disabling DWARF support, it
fixes it. This was reported as gcc PR68468.

In order to get C++ working without unresolved symbols, we also need to
disable symbol versioning (--disable-symvers). This is a remaining issue
in gcc which will be investigated at a later point.

Since this commit fixes C++ support in Blackfin, it re-enables the
selection of C++ support for this architecture.

Fixes:
  (alsa-lib emutls)
  http://autobuild.buildroot.net/results/8544ce58d75820666579db93a25ca5656a8efa8e/
  (cairo emutls)
  http://autobuild.buildroot.net/results/88b02a5dd5408318941ccbfcea0a9cbaa331500a/
  (audiofile c++)
  http://autobuild.buildroot.net/results/394e530c5dcd9ccb590eb151aeaadb37d11e0e39/
  (assimp c++)
  http://autobuild.buildroot.net/results/01f4be126c2d786a5ad7f220c2cf60539888a480/
  (bellagio c++)
  http://autobuild.buildroot.net/results/ada/ada44228bf13ec05382275bd6571396f5ba2b1f7/

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Tested-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>