libsys: link symbolic

[kib]

Summary

so that libc/libthr do not preempt libsys symbols, esp. errno-related

[jrtc27]

To confirm I understand this:

• Linking libsys.so without -Bsymbolic means the linker deems the various symbols preemptible
• If libc.so is earlier in the link map then libsys's unfiltered symbols will be preempted by libc.so's at run time

[brooks]

I can conform this fixes the issues I've seen with an WITH_BIND_NOW jail.

[kib]

In D44027#1004528, @jrtc27 wrote:

To confirm I understand this:

• Linking libsys.so without -Bsymbolic means the linker deems the various symbols preemptible
• If libc.so is earlier in the link map then libsys's unfiltered symbols will be preempted by libc.so's at run time

It also means that symbols looked up not _from the filter_ are preempted. This is the cause of the problem