gnu/lib/csu: Add -fno-asynchronous-unwind-tables to the CFLAGS.

gnu/lib/csu: Add -fno-asynchronous-unwind-tables to the CFLAGS.

Quoth FreeBSD's commit msg:

Often reported issue with newer ld is:
error in /usr/lib/crtendS.o(.eh_frame); no .eh_frame_hdr table will
be created.

The issue is that crtend is compiled with unwind table, and also it
places the special CIE into the .eh_frame indicating the end of
section, that is located before generated unwind table. New ld has
assertion that verifies that closing CIE is indeed the last CIE,
causing the crypting message to be issued, and refusing to generate
dwarf unwind.

Add -fno-asynchronous-unwind-tables to disable unwind table generation
for crtbegin/crtend.

Taken-from: FreeBSD