samples: zperf: Improve TCP performance

For TCP, CONFIG_PICOLIBC_USE_MODULE=y has a significant impact
on performance.

Use nucleo_h743zi board with loopback:
```
before:
tcp upload: 49.35 Mbps/s

after:
tcp upload: 56.52 Mbps/s
```

Use nucleo_h743zi board, default config:
```
before:
tcp upload: 71.3Mbps/s
tcp download: 75.11 Mbps/s

after:
tcp upload: 75.3Mbps/s
tcp download: 94.39 Mbps/s
```

Use nucleo_h743zi board, with tx async mode #93965:
```
before:
tcp upload: 71.3Mbps/s
tcp download: 75.11 Mbps/s

after:
tcp upload: 92.9Mbps/s
tcp download: 94.5Mbps/s
```

Signed-off-by: WenBin Zhang <[email protected]>

Author: Freey0

samples/net/zperf/prj.conf

@@ -44,3 +44,6 @@ CONFIG_SHELL_CMDS_RESIZE=n
 CONFIG_CACHE_MANAGEMENT=y
 CONFIG_SPEED_OPTIMIZATIONS=y
 CONFIG_TIMESLICING=n
+
+# For speed optimizations
+CONFIG_PICOLIBC_USE_MODULE=y