This is the exact same problem as described in Bug 161344.
Re-discovered this because libmpfr 4.0.2 fails most of its
tests unless one disables thread local storage. 

hydra$ uname -a 
FreeBSD hydra 12.0-RELEASE FreeBSD 12.0-RELEASE r341666 GENERIC  powerpc

hydra$ echo $CC
/usr/local/bin/gcc8

hydra$ echo $CFLAGS 
-g -m64 -std=c99 -fno-builtin -O0 -mcpu=970 -mcall-freebsd -mno-altivec -mno-float128 -mno-float128-hardware -mfull-toc -mregnames -fno-unsafe-math-optimizations

hydra$ cat -n t1.c
     1
     2  int f(int x) {
     3    return x;
     4  }
     5  struct s_t {
     6    int (*func)(int);
     7  };
     8  struct s_t __thread x = { f };
     9
    10
hydra$ 

hydra$ cat -n t2.c
     1
     2
     3  struct s_t {
     4    int (*func)(int);
     5  };
     6  extern struct s_t __thread x;
     7  int main(int argc, const char *argv[]) {
     8    return (*x.func)(argc);
     9  }
    10
hydra$ 

hydra$ $CC $CFLAGS -shared -o t1.so -fPIC t1.c
hydra$ $CC $CFLAGS -o foo t2.c t1.so
hydra$ LD_LIBRARY_PATH=. ./foo
Segmentation fault (core dumped)
hydra$

hydra$ gdb foo foo.core
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Reading symbols from foo...done.
[New LWP 100253]
Core was generated by `./foo'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x0000000010000a1c in main (argc=1, argv=0x3fffffffffffd7f0) at t2.c:8
8         return (*x.func)(argc);
(gdb) where
#0  0x0000000010000a1c in main (argc=1, argv=0x3fffffffffffd7f0) at t2.c:8
(gdb) list
3       struct s_t {
4         int (*func)(int);
5       };
6       extern struct s_t __thread x;
7       int main(int argc, const char *argv[]) {
8         return (*x.func)(argc);
9       }
10
(gdb) quit


However the problem goes away entirely with r344130.

Also tested the situation with mpfr 4.0.2 and TLS seems to work with r344130.