provide arch-generic fdpic self-relocation code for crt1 to use

this file is intended to be included by crt_arch.h on fdpic-based
targets and needs to be called from the entry point asm.

src/internal/fdpic_crt.h

+#include <stdint.h>
+
+__attribute__((__visibility__("hidden")))
+void *__fdpic_fixup(void *map, uintptr_t *a, uintptr_t *z)
+{
+	/* If map is a null pointer, the program was loaded by a
+	 * non-FDPIC-aware ELF loader, and fixups are not needed,
+	 * but the value for the GOT pointer is. */
+	if (!map) return (void *)z[-1];
+
+	struct {
+		unsigned short version, nsegs;
+		struct fdpic_loadseg {
+			uintptr_t addr, p_vaddr, p_memsz;
+		} segs[];
+	} *lm = map;
+	int nsegs = lm->nsegs, rseg = 0, vseg = 0;
+	for (;;) {
+		while (*a-lm->segs[rseg].p_vaddr >= lm->segs[rseg].p_memsz)
+			if (++rseg == nsegs) rseg = 0;
+		uintptr_t *r = (uintptr_t *)
+			(*a + lm->segs[rseg].addr - lm->segs[rseg].p_vaddr);
+		if (++a == z) return r;
+		while (*r-lm->segs[vseg].p_vaddr >= lm->segs[vseg].p_memsz)
+			if (++vseg == nsegs) vseg = 0;
+		*r += lm->segs[vseg].addr - lm->segs[vseg].p_vaddr;
+	}
+}