host-gcc: exclude -lgcc to fix -mx32 [qemu_]x86_64 regression

PR #9522 series ending with commit c2c9265b ("tests: cmsis: Disable
two cmsis portability tests on x86_64") added -mx32 support for the
x86_64 ARCH and qemu_x86_64. While this was implemented in
"compiler/gcc/target.cmake" as fall back from cross-compilation to the
host compiler, it worked with a direct ZEPHYR_TOOLCHAIN_VARIANT=host
too.

Later, -lgcc was added to "compiler/host-gcc/target.cmake" by PR #12674
to fix the -m32 x86 build. This broke the x86_64 build when using
ZEPHYR_TOOLCHAIN_VARIANT=host because even "multilib" packages usually
don't feature the -mx32 version of libgcc.

Fix this by excluding -lgcc in compiler/host-gcc/target.cmake just like
compiler/gcc/target.cmake always did for x86_64.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>