with start/stop (or attach), existing threads do not use the proper syscall method

Running a forthcoming test api.detach, modeled after api.startstop, on 32-bit Linux, for #95:

Here's vsyscall:

```
interp: start_pc = 0xf779cbe0
0xf779cbe0 51 push %ecx %esp -> %esp 0xfffffffc(%esp)[4byte]
0xf779cbe1 52 push %edx %esp -> %esp 0xfffffffc(%esp)[4byte]
0xf779cbe2 55 push %ebp %esp -> %esp 0xfffffffc(%esp)[4byte]
0xf779cbe3 89 e5 mov %esp -> %ebp
0xf779cbe5 0f 34 sysenter -> %esp
end_pc = 0xf779cbe7

Fragment 26, tag 0xf779cbe0, flags 0x1801030, shared, size 10, must end trace:
0x55b95308 51 push %ecx %esp -> %esp 0xfffffffc(%esp)[4byte]
0x55b95309 52 push %edx %esp -> %esp 0xfffffffc(%esp)[4byte]
0x55b9530a 55 push %ebp %esp -> %esp 0xfffffffc(%esp)[4byte]
0x55b9530b 89 e5 mov %esp -> %ebp
0x55b9530d e9 27 fe 01 00 jmp $0x55bb5139
-------- exit stub 0: -------- type: fall-through/speculated/IAT
0x55bb5139 67 64 a3 00 00 addr16 mov %eax -> %fs:0x00[4byte]
0x55bb513e b8 d8 90 b6 55 mov $0x55b690d8 -> %eax
0x55bb5143 e9 b8 8d fd ff jmp $0x55b8df00
```

Thread 3 invoking futex and using OP_sysenter in thread-private do_syscall,so when it gets a signal
interrupting the syscall is says the PC is on the vsyscall page at the sysenter resumption point:

```
do_syscall:
0x55cf6200 cd 80 int $0x80
0x55cf6202 a3 1c 5a c9 55 mov %eax -> 0x55c95a1c[4byte]
0x55cf6207 b8 6c fa 2a f7 mov $0xf72afa6c -> %eax
0x55cf620c e9 ef fc ff ff jmp $0x55cf5f00

Just updated syscall routine:
0x55cf6200 0f 34 sysenter -> %esp

cache_enter = 0x55b8de80, target = 0x55b95308
Exit from F26(0xf779cbe0).0x55b9530d (shared)
(block ends with syscall)
Entry into do_syscall to execute a non-ignorable system call
system call 240
fcache_enter = 0x55cf5e80, target = 0x55cf6200

master_signal_handler: sig=12, retaddr=0xf779cbd0
siginfo: sig = 12, pid = 30715, status = 0, errno = 0, si_code = -6
xip=0xf779cbe9
```

Thread 4 is similar except the signal says it's at the post-OP_int in do_syscall:

```
do_syscall:
0x55d3e200 cd 80 int $0x80
0x55d3e202 a3 9c 6e c9 55 mov %eax -> 0x55c96e9c[4byte]
0x55d3e207 b8 6c fa 2a f7 mov $0xf72afa6c -> %eax
0x55d3e20c e9 ef fc ff ff jmp $0x55d3df00

fcache_enter = 0x55b8de80, target = 0x55b95308
Exit from F26(0xf779cbe0).0x55b9530d (shared)
(block ends with syscall)
Entry into do_syscall to execute a non-ignorable system call
system call 240
fcache_enter = 0x55d3de80, target = 0x55d3e200

master_signal_handler: sig=12, retaddr=0xf779cbd0
siginfo: sig = 12, pid = 30715, status = 0, errno = 0, si_code = -6
xip=0x55d3e202
```

The syscall method is exactly the reason -- the later threads all have
sysenter, but the earlier ones are not updated when the method is
"discovered", only the current thread is updated:

```
# grep -A 1 ^do_syscall: `ls -1td *0|head -1`/*
api.detach.0.30715.html:do_syscall:
api.detach.0.30715.html- 0x55b8ea00 cd 80 int $0x80

log.0.30715.html:do_syscall:
log.0.30715.html- 0x55bfe200 cd 80 int $0x80

log.10.30722.html:do_syscall:
log.10.30722.html- 0x55f0e200 0f 34 sysenter -> %esp

log.1.30718.html:do_syscall:
log.1.30718.html- 0x55c46200 cd 80 int $0x80

log.2.30716.html:do_syscall:
log.2.30716.html- 0x55c8e200 cd 80 int $0x80

log.3.30719.html:do_syscall:
log.3.30719.html- 0x55cf6200 cd 80 int $0x80

log.4.30723.html:do_syscall:
log.4.30723.html- 0x55d3e200 cd 80 int $0x80

log.5.30721.html:do_syscall:
log.5.30721.html- 0x55d86200 0f 34 sysenter -> %esp

log.6.30724.html:do_syscall:
log.6.30724.html- 0x55dce200 0f 34 sysenter -> %esp

log.7.30720.html:do_syscall:
log.7.30720.html- 0x55e16200 0f 34 sysenter -> %esp

log.8.30725.html:do_syscall:
log.8.30725.html- 0x55e5e200 0f 34 sysenter -> %esp

log.9.30717.html:do_syscall:
log.9.30717.html- 0x55ea6200 0f 34 sysenter -> %esp
```

This is unique to the start/stop interface, or to attach, b/c in regular
full control mode we'd see a syscall prior to a 2nd thread.

This causes several problems, such as state translation and safe spot
checking getting confused.

A related issue is that we're not translating when sysenter is the method
and a suspended thread is at do_syscall. Xref a84214e which should be
reverted and a proper translation solution put in place: we do need to
translate when at do_syscall when the method is sysenter, both when
do_syscall holds an int or a sysenter, for detach. This can happen w/o
this mixed-do_syscall if a thread just hasn't entered the kernel yet, but
is much more likely with the mixed.