FTBFS: Building for RISC-V with --disable-tcg fails
## Host environment
- Operating system: Ubuntu 26.04
- OS/kernel version: 6.18.3
- Architecture: riscv64, RVA23U64, SpacemiT K3
- QEMU flavor: riscv64-softmmu
- QEMU version: HEAD, ee7eb612be8f
## Build failure
Building fails with
```
./configure --prefix=/usr/local --enable-kvm --disable-tcg --target-list="riscv64-softmmu"
make
```
with
```
:: ../target/riscv/csr.c:2134:21: error: implicit declaration of function ‘cpu_unwind_state_data’ [-Werror=implicit-function-declaration]
:: 2134 | if (ra == 0 || !cpu_unwind_state_data(env_cpu(env), ra, data)) {
:: | ^~~~~~~~~~~~~~~~~~~~~
:: ../target/riscv/csr.c:2119:21: error: nested extern declaration of ‘cpu_unwind_state_data’ [-Werror=nested-externs]
```
## Analysis
The function declaration in include/exec/cpu-common.h is guarded by
```
#ifdef CONFIG_TCG
```
which makes sense as the implementation is in accel/tcg/translate.all.
Target i386 uses
```
#ifdef CONFIG_TCG
#else
qemu_build_not_reached();
#endif
```
to guard the use of cpu_unwind_state_data(). Such a guard is missing for the RISC-V target.
issue