arch/riscv64: raise the baseline to rva20u64 ...

... when Rust LTO is enabled to workaround a behavior change in LLVM 20
where the default baseline does not have +a

Author: liushuyu

arch/riscv64.sh

@@ -7,9 +7,5 @@ if ! bool "$USECLANG" && [ "$(echo __GNUC__ | $CC -E -xc - | tail -n 1)" -ge 14
 	CFLAGS_COMMON_ARCH=('-mno-omit-leaf-frame-pointer')
 fi
 LDFLAGS_COMMON_CROSS=('-Wl,-rpath -Wl,/usr/lib -Wl,-rpath-link' '-Wl,/var/ab/cross-root/riscv64/usr/lib' '-L/var/ab/cross-root/riscv64/usr/lib')
-# FIXME: Since Rustc 1.88 (built against LLVM 20), RISC-V executables
-# would fail to link due to missing -latomic symbols.
-#
-# This may be an upstream issue.
-RUSTFLAGS_COMMON_ARCH=('-Clink-arg=-mabi=lp64d' '-Clink-arg=-latomic')
-RUSTFLAGS_COMMON_ARCH_LTO=("${RUSTFLAGS_COMMON_ARCH[@]}" '-Clink-arg=-Wl,-mllvm=-mattr=+d' '-Clink-arg=-latomic')
+RUSTFLAGS_COMMON_ARCH=("-Clink-arg=-mabi=lp64d")
+RUSTFLAGS_COMMON_ARCH_LTO=("${RUSTFLAGS_COMMON_ARCH[@]}" '-Clink-arg=-Wl,-mllvm=-mattr=+rva20u64')