fix 32 bit int simple return of riscv64 bridge

fix 32 bit int simple return of riscv64 bridge

Sometimes we need to return a 32 bit integer into a 64 bit integer.
For example, in pyuno.cxx:PyUNO_bool(), an int(32bit) is returned in
type Py_ssize_t(64bit). We assume that this 32bit int was put in low
32 bit of register a0. The bridge may return with high 32 bit
uncleaned and compiler might directly bind this register to 64 bit
variable in error.

This bug produces when build pyuno with gcc-12 with -O2.
https://bugs.documentfoundation.org/show_bug.cgi?id=155937
https://lists.debian.org/debian-riscv/2023/07/msg00011.html

So we need to clean the high 32 bit in bridge.

Change-Id: I37aafb03ba9523cfb90912871308921fbeaf5f0c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/154837
Tested-by: René Engelhard <rene@debian.org>
Reviewed-by: René Engelhard <rene@debian.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Gbp-Pq: Name fix-riscv64-bridges.diff