configs/andes_ae350_45: Fix build failure caused by Zicsr and Zifencei

Don't specify the ARCH_FLAGS of U-Boot, so both extensions can be
appended to -march when building w/ gcc-12.

Fixes:
- https://gitlab.com/buildroot.org/buildroot/-/jobs/4839059655

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

configs/andes_ae350_45_defconfig

@@ -38,7 +38,6 @@ BR2_TARGET_UBOOT_NEEDS_OPENSBI=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME="u-boot.itb"
 BR2_TARGET_UBOOT_SPL=y
-BR2_TARGET_UBOOT_CUSTOM_MAKEOPTS="ARCH_FLAGS=-march=rv64imafdc"
 BR2_PACKAGE_HOST_DOSFSTOOLS=y
 BR2_PACKAGE_HOST_GENIMAGE=y
 BR2_PACKAGE_HOST_MTOOLS=y