Summary:
Changes jitlog to use a memfd, an anonymous in memory file descriptor, rather
than a file on disk. Also clean up this fd at the end of an OID run rather than
leaving it in the hope it's valid next time.
A previous attempt to land this used a `char*` from the OID process space in
the remote target syscall. Somehow this works with our integration test target,
but not globally. Changed to use the previous behaviour of putting the syscall
arg in the empty text segment. In doing this I noticed that the text segment
wouldn't be initialised at this point on a fresh process, so we were copying
into effectively an uninitialised address. Move the jit log fd setup to after
the segment setup accordingly.
Test plan:
- CI
- Tested on an integration test target as before. Works.
- Created a new target that definitely doesn't have this string in (simple for
loop). Failed before, works now.
Example:
```sh
$ OID_TEST_ARGS='-fjit-logging' stest OidIntegration.simple_struct
...
I1121 02:57:36.136890 500897 OIDebugger.cpp:269] Outputting JIT logs:
I1121 02:57:36.136896 500897 OIDebugger.cpp:272] JITLOG: SimpleStruct
@00007ffc639be180
I1121 02:57:36.136899 500897 OIDebugger.cpp:272] JITLOG: a @00007ffc639be180
I1121 02:57:36.136901 500897 OIDebugger.cpp:272] JITLOG: obj @00007ffc639be180
I1121 02:57:36.136904 500897 OIDebugger.cpp:272] JITLOG: b @00007ffc639be184
I1121 02:57:36.136905 500897 OIDebugger.cpp:272] JITLOG: obj @00007ffc639be184
I1121 02:57:36.136907 500897 OIDebugger.cpp:272] JITLOG: c @00007ffc639be188
I1121 02:57:36.136909 500897 OIDebugger.cpp:272] JITLOG: obj @00007ffc639be188
I1121 02:57:36.136911 500897 OIDebugger.cpp:278] Finished outputting JIT logs.
...
```
