gdb: update corner case when canonicalizing riscv syscall names

gdb: update corner case when canonicalizing riscv syscall names

The script syscalls/riscv-canonicalize-syscall-gen.py has been recently
introduced to help support record-full in riscv systems.  However, it
was developed before commit 432eca4113d5748ad284a068873455f9962b44fe,
which made the GDB enum more consistent, which forced the python script
to have a corner case for the "gdb_old_mmap" case.

Since the aforementioned commit has already been merged, we need to
update the special case for the mmap syscall. A special case is still
needed because the script would expect that the glibc sources call the
syscall "old_mmap", or that gdb call it "gdb_sys_mmap", neither of which
happens unfortunately.

This commit doesn't change the .c file because it was already fixed by a
different commit, 65ab41b7d5c612b6000b28f4c50bb256b2a9e22b, which was
pushed as obvious to fix the build issues.

Approved-By: Tom Tromey <tom@tromey.com>