[CMake] Enable -z defs on Solaris

[CMake] Enable -z defs on Solaris

I wondered why I hadn't seen the link failures in D130571
<https://reviews.llvm.org/D130571> on Solaris/sparcv9 and found that
Solaris, unlike Linux, doesn't use `-Wl,-z,defs` when linking shared
objects.  The three affected shared objects indeed have undefined
references to several 64-bit atomics.

This patch will expose such issues in the future by defaulting to
`-Wl,-z,defs` on Solaris, as recommended in `ld(1)`.

Tested on `sparcv9-sun-solaris2.11` and `amd64-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D130572