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

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>