gdb: fix building with all targets

gdb: fix building with all targets

Commit b9c7eed0c2409fc640129a38d80a2bf1212b464a recently introduced
a build failure, because the file gdb/riscv-canonicalize-syscall-gen.c
hasn't been added to the ALL_64_TARGET_OBS variable in the makefile,
leading to a linker issue.  This commit fixes that.

Also, turns out, the new file was slightly outdated, as the gdb_old_mmap
syscall has been renamed to gdb_sys_old_mmap in commit
432eca4113d5748ad284a068873455f9962b44fe.  This commit also fixes that
on the generated file itself, to quickly fix the build. A followup
commit will fix the python file responsible for generating the .c file.