cmake: Revert breaking change to CMAKE_SYSTEM_NAME

This commit reverts a breaking change in CMAKE_SYSTEM_NAME introduced by
"Zephyr" back to "Generic") and removes the file
`cmake/modules/Platform/Zephyr`.

Both changes in the aforementioned PR were only introduced to ultimately
modify the value of the global CMake property TARGET_SUPPORTS_SHARED_LIBS
for the special case of building for Xtensa with LLEXT.

The modification of CMAKE_SYSTEM_NAME is considered a breaking change
because it has the potential to alter the build of any non-trivial project
that previously checked for the "Generic" system identifier as
corresponding to Zephyr - for example by doing
`if (CMAKE_SYSTEM_NAME STREQUAL "Generic")`. Such builds may now break in
many ways including silently when there is no `else()` clause with a
`message()` to alert the user that a whole configuration block had been
skipped.

In essence, that CMAKE_SYSTEM_NAME modification was only introduced in
order to have CMake to load `cmake/modules/Platform/Zephyr.cmake` which in
turn adjusted the value of TARGET_SUPPORTS_SHARED_LIBS.

But the use of a CMake platform file like this is ineffective for
non-trivial projects where one or more top level CMake `project()` calls
may happen before the first call to `find_package(Zephyr)` because in such
cases CMAKE_MODULE_PATH will not have been modified yet to contain the
path to <Zephyr_ROOT>/cmake/modules and thus no platform file will be
include by CMake.

This patch moves the conditional override of TARGET_SUPPORTS_SHARED_LIBS
needed by some archs (e.g. Xtensa) into the `kernel.cmake` module which
is known to be the first call to `project()` that enables any language and
thus the one that must come before any artifact target can be defined.

Note commit 64e7d85e added a Kconfig to specify the object type of llext
being built, so it's not tied to the arch anymore but to the
CONFIG_LLEXT_TYPE_ELF_SHAREDLIB option.

Signed-off-by: Nicolas Lebedenco <nicolas@lebedenco.net>