This sbz-golang-freebsd-ptrace.diff is an *alpha* patch to start ptrace support 
on FreeBSD amd64 platform. So it exports ptrace API in the syscall package 
using cgo and regenerate z* files using commands below in src/pkg/syscall
directory:

% arch=$(sysctl -n hw.machine) mkerrors.sh -m${arch:3}.
% mkall.sh -n

On my system below:

FreeBSD ogoshi.lan 10.0-CURRENT FreeBSD 10.0-CURRENT #4 r251383M: Tue Jun  4 18:17:24 CEST 2013     root@ogoshi.lan:/usr/obj/usr/home/sbz/freebsd/svn/src/sys/GENERIC  amd64

Install lang/go:
% sudo make -C /usr/ports/lang/golang install clean
% fetch ptrace.go it does exactly the same as ptrace.c without PT_GETREGS part.

Before:
% /usr/local/go/bin/go run ptrace.go
# command-line-arguments
./ptrace.go:17: undefined: syscall.PtraceRegs
./ptrace.go:50: undefined: syscall.PtraceRegs
./ptrace.go:57: undefined: syscall.PtraceAttach
./ptrace.go:61: undefined: syscall.PtraceSingleStep
./ptrace.go:64: undefined: syscall.PtraceGetRegs
./ptrace.go:76: undefined: syscall.PtraceDetach

Apply patch from upstream:
% mkdir ~/tmp
% cd ~/tmp && hg clone https://code.google.com/p/go/
% cd go && patch -p1 ./sbz-golang-freebsd-ptrace.diff

See patch.log for details. Then rebuild it:

% cd src && sh make.bash

See build.log for details.

After:
% ~/tmp/go/bin/go run /tmp/ptrace.go
usage: /tmp/go-build929942693/command-line-arguments/_obj/exe/ptrace <pid>
exit status 1

Trying to attach an already launched pid:

% xterm &
[1] 46757
% ~/tmp/go/bin/go run /tmp/ptrace.go $!
[+] Attaching pid 46757
[+] SingleStep pid 46757
[+] GetRegs pid 46757
R15:	0
R14:	0
R13:	0
R12:	0
R11:	0
R10:	0
R9:	0
R8:	0
RDI:	0
RSP:	0
RSI:	0
RBP:	0
RBX:	0
RCX:	0
RAX:	0
RIP:	0
RFLAGS:	0
TRAPNO:	0
ERR:	0
FS:	0
GS:	0
ES:	0
DS:	0
CS:	0
SS:	0
[+] Detaching pid 46757

Status: WIP
 - ATTACH 	[OK]
 - DETACH 	[OK]
 - STEP		[OK]
 - GETREGS 	[KO]
 - ...