fix:[kstdio]Disable "-Wimplicit-fallthrough" below GNUC V7 #3708

Job	Run time
RT-Thread Online Packages (STM32F407 RT-Spark)	5m 9s
RTduino/Arduino Libraries (STM32F412 Nucleo)	13m 19s
RTduino/Arduino Libraries (Raspberry Pico)	10m 48s
others_at32_hc32_ht32	5m 34s
stm32l4_f0_f1	12m 11s
stm32_f2_f4	7m 47s
stm32_f7_g0_h7_mp15_u5_h5_wb5	5m 41s
nxp_renesas	4m 57s
gd32_n32_apm32	4m 42s
nordic_Infineon_TI_microchip	6m 44s
mips	2m 44s
aarch64	3m 24s
riscv-none	3m 40s
riscv64-unknown	3m 15s
hpmicro	2m 17s
llvm-arm	4m 53s
simulator	2m 19s
ESP32C3	4m 42s
i386-unknown	3m 28s
	1h 47m 34s

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