toolchain/ghs: fix the ghs build warning

CXX:  libcxxmini/libxx_new.cxx cxarm: cxarm: cxarm: cxarm: Warning: cxarm: cxarm: Unknown option "--gnu++17" passed to linker
Warning: Warning: Warning: Warning: cxarm: Unknown option "--gnu++17" passed to linker
Unknown option "--gnu++17" passed to linker
Warning: Unknown option "--gnu++17" passed to linker
Unknown option "--gnu++17" passed to linker
Unknown option "--gnu++17" passed to linker
Warning: Unknown option "--gnu++17" passed to linker
CXX:  libcxxmini/libxx_newa.cxx cxarm: Warning: Unknown option "--gnu++17" passed to linker

Signed-off-by: guoshichao <[email protected]>

Author: extinguish

boards/arm/mps/mps2-an500/configs/nsh/defconfig

@@ -23,6 +23,7 @@ CONFIG_CMSDK_UART0_RX_IRQ=16
 CONFIG_CMSDK_UART0_SERIAL_CONSOLE=y
 CONFIG_CMSDK_UART0_TX_IRQ=17
 CONFIG_CMSDK_UART=y
+CONFIG_CXX_STANDARD="c++17"
 CONFIG_DEBUG_ASSERTIONS=y
 CONFIG_DEBUG_FEATURES=y
 CONFIG_DEBUG_SCHED=y