cmake: SHELL: vs zephyr_get_compile_X_for_lang

In zephyr_get_XYZ_for_lang() functions in extensions.cmake we try to mimic
what cmake does in generation time to filter out SHELL: tags. However,
this filtering was only done for list entries that did not contain
generator expressions.

This causes broken command lines for e.g. e.g. for the kobject_hash*c
if the toolchain puts "SHELL: $<$<COMPILER_LANGUAGE:C>:some thing>"
into the options.

Signed-off-by: Björn Bergman <bjorn.bergman@iar.com>