[Driver] Improve legibility of ld -z options on Solaris

Following the lead of the Linux code, this patch passes the `ld -z` options
as two separate args on Solaris, improving legibility.  For lack of a
variadic `std::push_back`, `getAsNeededOption` had to be changed to
`addAsNeededOption`, matching other `add*Options` functions, changing
callers accordingly.  The additional args are also used in a WIP revision
of the Solaris GNU ld patch D85309 <https://reviews.llvm.org/D85309>, which
will allow runtime selection of the linker to use.

Tested on `amd64-pc-solaris2.11` and `x86_64-pc-linux-gnu`.

Differential Revision: https://reviews.llvm.org/D158955