target/riscv: gdbstub: Turn on debugger mode before calling CSR predicate()

target/riscv: gdbstub: Turn on debugger mode before calling CSR predicate()

Since commit 94452ac4cf26 ("target/riscv: remove fflags, frm, and fcsr from riscv-*-fpu.xml")
the 3 FPU CSRs are removed from the XML target decription. The
original intent of that commit was based on the assumption that
the 3 FPU CSRs will show up in the riscv-csr.xml so the ones in
riscv-*-fpu.xml are redundant. But unforuantely that is not true.
As the FPU CSR predicate() has a run-time check on MSTATUS.FS,
at the time when CSR XML is generated MSTATUS.FS is unset, hence
no FPU CSRs will be reported.

The FPU CSR predicate() already considered such a case of being
accessed by a debugger. All we need to do is to turn on debugger
mode before calling predicate().

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230228104035.1879882-12-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>