external-csl-toolchain: fixes for metadata changes

external-csl-toolchain: fixes for metadata changes

These allow the recipe to build again:

- add compilerlibs, g++, libgcc to the provides
- add linux-libc-headers-dev to the packages
- in libc-package, only sed the ldd.bash.in file if it exists, as the external
  toolchain is using that class as well
- shift the inherit location of the libc classes, as they were overriding the
  recipe's do_install
- use ?= for EXTERNAL_TOOLCHAIN, so the user can set it

(From OE-Core rev: 8fb1c84f4cadf8d7a061fd6d90d270c19b474bfe)

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>