gdb/record: Speeding up recording in RISC-V

I measured that removing saving mem chunks and regs to std::vector before
calling API functions speeds up stepping up to 15%, so I added this
optimization (as Guinevere Larsen <guinevere@redhat.com> recommended in
initial support). It turns out that after this, the m_record_type and
m_error_occured no longer needed, so I removed them too.

Approved-By: Guinevere Larsen <guinevere@redhat.com>