riscv, gdbstub.c: fix reg_width in ricsv_gen_dynamic_vector_feature()

riscv, gdbstub.c: fix reg_width in ricsv_gen_dynamic_vector_feature()

Commit 33a24910ae changed 'reg_width' to use 'vlenb', i.e. vector length
in bytes, when in this context we want 'reg_width' as the length in
bits.

Fix 'reg_width' back to the value in bits like 7cb59921c05a
("target/riscv/gdbstub.c: use 'vlenb' instead of shifting 'vlen'") set
beforehand.

While we're at it, rename 'reg_width' to 'bitsize' to provide a bit more
clarity about what the variable represents. 'bitsize' is also used in
riscv_gen_dynamic_csr_feature() with the same purpose, i.e. as an input to
gdb_feature_builder_append_reg().

Cc: Akihiko Odaki <akihiko.odaki@daynix.com>
Cc: Alex Bennée <alex.bennee@linaro.org>
Reported-by: Robin Dapp <rdapp.gcc@gmail.com>
Fixes: 33a24910ae ("target/riscv: Use GDBFeature for dynamic XML")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Message-ID: <20240517203054.880861-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
(cherry picked from commit 583edc4efb7f4075212bdee281f336edfa532e3f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>