tests: Fix the compile warning for arc

the following compile warning will fail the CI:

/zephyr/tests/kernel/fp_sharing/generic/src/float_regs_arc_gcc.h:
41:8: error: /unused variable 'temp' [-Werror=unused-variable]

/zephyr/tests/kernel/fp_sharing/generic/src/float_regs_arc_gcc.h:
75:8: error: /unused variable 'temp' [-Werror=unused-variable]

cc1: all warnings being treated as errors

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>