cmake: Delay computation of linker paths until needed

With inclusion of the optimization flag into the multilib selection
process, we cannot compute the compiler library path when the compiler's
target.cmake is processed as LINKER_OPTIMIZATION_FLAG is not computed until
much later.

Instead, add a function (compiler_file_path) which can be used to locate
the appropriate crtbegin.o and crtend.o files.

Delay computation of lib_include_dir and rt_library until after all
compiler flags have been computed by adding compiler_set_linker_properties
and calling that just before toolchain_linker_finalize is invoked.

Place default implementations of both of these functions in a new file,
cmake/compiler/target_template.cmake, where we assume the compiler works
like gcc or clang and handlers the --print-file-name and
--print-libgcc-file-name options. Compilers needing alternate
implementations can override these functions in their target.cmake files.

These implementations require that no generator expressions are necessary
for the compiler to compute the right library paths.

This mechanism is also used to take any additional compiler options by
adding a new (optional) linker function,
toolchain_linker_add_compiler_options, which maps compiler options to
equivalent linker options, discarding any that aren't applicable.

Signed-off-by: Keith Packard <keithp@keithp.com>