Previously we used separate placeholders for builtin linker flags:
* CMAKE_<LANG>_LINK_FLAGS for EXECUTABLEs
* CMAKE_SHARED_LIBRARY_CREATE_<LANG>_FLAGS for SHARED libraries
* CMAKE_SHARED_MODULE_CREATE_<LANG>_FLAGS for MODULE libraries
These are now always replaced by the empty string, so drop them
from our rule variables.
This was done by mistake in commit d090159318 (ENH: add support for the
ADSP toolchains for Blackfin, Shark and TigerShark DSPs, patch from
Raphael Cotty, 2007-07-12, v2.6.0~1446), and preserved in the
generalization by commit e9eabb0dcd (ADSP: Configure compiler in
compiler module, 2022-03-24, v3.24.0-rc1~331^2~1). Fix both places.
This variable define how the link step is done. Possible values are:
* DRIVER: the compiler is used as driver for the link step
* LINKER: the linker is used directly for the link step.
