ARC: Make vfork weak in libc

ARC: Make vfork weak in libc

Anton reported:

------------------->8---------------------
Package iperf cannot be built in Buildroot due to following error:

/home/akolesov/env/autobuild/tmp/host/usr/bin/arc-buildroot-linux-uclibc-g++
-Wall -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64
-pipe -Os    --static -o iperf -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE
-D_FILE_OFFSET_BITS=64  -pipe -Os   -pthread  -DHAVE_CONFIG_H Client.o
Extractor.o Launch.o List.o Listener.o Locale.o PerfSocket.o ReportCSV.o
ReportDefault.o Reporter.o Server.o Settings.o SocketAddr.o gnu_getopt.o
gnu_getopt_long.o main.o service.o sockets.o stdio.o tcp_window_size.o
../compat/libcompat.a
/home/akolesov/env/autobuild/tmp/host/usr/arc-buildroot-linux-uclibc/sysroot/usr/lib/libc.a(vfork.os):
In function `vfork':
(.text+0x0): multiple definition of `vfork'
/home/akolesov/env/autobuild/tmp/host/usr/arc-buildroot-linux-uclibc/sysroot/usr/lib/libpthread.a(ptfork.os):ptfork.c:(.text+0xc0):
first defined here
collect2: error: ld returned 1 exit status
------------------->8---------------------

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Anton Kolesov <Anton.Kolesov@synopsys.com>
Cc: Francois Bedard <Francois.Bedard@synopsys.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>