Support R_X86_64_TLSGD relocation on FreeBSD

The FreeBSD C <ctype.h> header supports per-thread locales by exporting a
static inline function that references the `_ThreadRuneLocale` thread-local
variable.  This means that object files that use e.g. isdigit(3) end up with
TLSGD(19) relocations, and would not load into ghci or the language server.

Here we add support for this type of relocation, for now just on FreeBSD, and
only for external references to thread-specifics defined in already loaded
dynamic modules (primarily libc.so).  This is sufficient to resolve the
<ctype.h> issues.

Runtime linking of ".o" files which *define* new thread-specific variables
would be noticeably more difficult, as this would likely require new rtld APIs.