target/riscv/cpu_helper: Fix linking problem with semihosting disabled

If QEMU has been configured with "--without-default-devices", the build
is currently failing with:

 /usr/bin/ld: libqemu-riscv32-softmmu.a.p/target_riscv_cpu_helper.c.o:
  in function `riscv_cpu_do_interrupt':
 .../qemu/target/riscv/cpu_helper.c:1678:(.text+0x2214): undefined
  reference to `do_common_semihosting'

We always want semihosting to be enabled if TCG is available, so change
the "imply" statements in the Kconfig file to "select", and make sure to
avoid calling into do_common_semihosting() if TCG is not available.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240906094858.718105-1-thuth@redhat.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/Kconfig

@@ -1,9 +1,9 @@
 config RISCV32
     bool
-    imply ARM_COMPATIBLE_SEMIHOSTING if TCG
+    select ARM_COMPATIBLE_SEMIHOSTING if TCG
     select DEVICE_TREE # needed by boot.c
 
 config RISCV64
     bool
-    imply ARM_COMPATIBLE_SEMIHOSTING if TCG
+    select ARM_COMPATIBLE_SEMIHOSTING if TCG
     select DEVICE_TREE # needed by boot.c

target/riscv/cpu_helper.c

@@ -1674,10 +1674,12 @@ void riscv_cpu_do_interrupt(CPUState *cs)
     if (!async) {
         /* set tval to badaddr for traps with address information */
         switch (cause) {
+#ifdef CONFIG_TCG
         case RISCV_EXCP_SEMIHOST:
             do_common_semihosting(cs);
             env->pc += 4;
             return;
+#endif
         case RISCV_EXCP_LOAD_GUEST_ACCESS_FAULT:
         case RISCV_EXCP_STORE_GUEST_AMO_ACCESS_FAULT:
         case RISCV_EXCP_LOAD_ADDR_MIS: