As there's no __attribute__((preconstructor)), you can just mush the code into the relevant section using some section attributes e.g.

#include <stdio.h>

int v;
int w;
int x;

__attribute__((constructor)) static void
foo(void)
{
    printf("Foo %d %d %d\n", v, w, x);
}

static void
bar(void)
{
    v = 3;
}

static void
bar1(void)
{
    w = 2;
}

static void
bar2(void)
{
    x = 1;
}

__attribute__((section(".preinit_array"))) static void (*y[])(void) = { &bar, &bar1, &bar2 };

int
main(int argc, char **argv)
{
    printf("Hello World\n");
}

File dumped into foo.c, compiled using: gcc -o foo foo.c, and then run yields an output of:

Foo 3 2 1
Hello World

File compiled using gcc -static -o foo foo.c, and then run yields the same output, so it does appear to work with statically linked binaries.

It will not work with .so files, though; the linker complains with:

/usr/bin/ld: /tmp/ccI0lMgd.o: .preinit_array section is not allowed in DSO
/usr/bin/ld: failed to set dynamic section sizes: Nonrepresentable section on output

I'd be inclined to avoid it, as code run in that section precedes all other initialization routines. If you're trying to perform some 'this is supposed to run first' initialization, then it's really not a good idea - you're just fighting a race condition which should be solved by some other mechanism.