cmake: toolchain: cache property GNULD_LINKER_IS_BFD

Cache the property GNULD_LINKER_IS_BFD between cmake invocations.
It is observed that, in repeated builds (2nd time and later),
this property becomes true even for non-bfd compatible linker.
So cache it to avoid any surprises.

Fixes #56501

Signed-off-by: Daniel Leung <daniel.leung@intel.com>