libdtrace: Change the binding of USDT probe symbols to STB_WEAK

Otherwise, if multiple object files contain references to the same
probe, newish lld will refuse to link them by default, raising a
duplicate global symbol definition error.  Previously, duplicate global
symbols with identical absolute st_values were permitted by both lld and
GNU ld.

Since dtrace has no use for probe function symbols after the relocation
performed by dtrace -G, make the symbols weak as well, following a
suggestion from MaskRay.

Reported by:	dim
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation

(cherry picked from commit be39466a10)

cddl/contrib/opensolaris/lib/libdtrace/common/dt_link.c

@@ -1612,6 +1612,7 @@ process_obj(dtrace_hdl_t *dtp, const char *obj, int *eprobesp)
 			 * invocation.
 			 */
 			if (rsym.st_shndx != SHN_ABS) {
+				rsym.st_info = GELF_ST_INFO(STB_WEAK, STT_FUNC);
 				rsym.st_shndx = SHN_ABS;
 				(void) gelf_update_sym(data_sym, ndx, &rsym);
 			}