cmake: detect sysroot for cross-compile toolchains

Fixes: #49587

Try to detect sysroot for cross-compile toolchain if not specified by
the user with `-DSYSROOT_DIR=<path>`.

First, the C compiler is asked if it knows its sysroot, some C compilers
are able to return the sysroot, as an example:
> $ arm-none-eabi-gcc -print-sysroot
> <path>/bin/../arm-none-eabi
> $

but majority of gcc-based C compilers seems to return empty string:
> $ arm-zephyr-eabi-gcc --print-sysroot
> $

in such cases, the cross-compiler target CMake file will try to discover
the sysroot by searching for libc.a, starting one level up from the
compiler location.

If no sysroot candidate is found, a warning will be printed to the user.
If a single sysroot candidate is found, this candidate will be selected.
If multiply sysroot candidates are found, a warning is printed, and the
first candiate in the list is selected. User may select another
candidate with `-DSYSROOT_DIR=<use-this>`.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>