cmake: gcc: Use --sysroot with gcc when SYSROOT_DIR is provided

When SYSROOT_DIR is provided, gcc should use it through the --sysroot=
option otherwise some commands won't work as expected.

For example, in the Yocto environment when cross compiling,
--print-libgcc-file-name prints only "libgcc.a" instead of the full
path to it and the subsequent assert_exists(LIBGCC_FILE_NAME) will
fail.

Fixes #45578
Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>