64 bit recording fixes part 1

This is the first set of fixes (out of many, I'm sure) coming from attempting to run a 64-bit `rr record` on a simple test program.  (There are some minor changes necessary to do that which are not included here: making sure `Registers` returns the correct architecture, compiling `librrpreload` as PIC, and removing `-m32` from compilation flags.  Those changes aren't appropriate at this time.)

Some of the fixes are pedestrian (the syscallname fixes, for instance).  Some are eliminating instances of 32-bit-ness.

I'm not entirely sure the syscall handling in the last several commits is correct.  In particular, I'd like feedback on whether turning unsupported syscalls into syscalls supported solely on x86-64 seems OK.

I think the definition of `arch_prctl` is correct, since we're just setting values that should be reflected in the registers we get back from ptrace.  It's possible that a `before_syscall_return` hook for that is needed...