Enforce usage -zrelax=transtls when GNU C Compiler is used on Solaris

When GNU C Compiler is used on Solaris (11), the Thread Local Storage is completely broken. The behaviour doesn't manifest when GNU ld is used. Thus, we need to enforce usage of GNU ld when GNU C Compiler is the compiler of choice.

For more background for this change, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90912

(cherry picked from commit d5842236)

Closes #1095 (closed)

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