ARC: Make vfork weak in libc

Anton reported:

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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
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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>

libc/sysdeps/linux/arc/vfork.S

@@ -15,7 +15,7 @@
 #define CLONE_VFORK		0x00004000
 #define CLONE_FLAGS_FOR_VFORK	(CLONE_VM|CLONE_VFORK|SIGCHLD)
 
-ENTRY(vfork)
+ENTRY(__vfork)
 	mov	r0, CLONE_FLAGS_FOR_VFORK
 	mov_s	r1, sp
 	mov	r8, __NR_clone
@@ -25,5 +25,7 @@ ENTRY(vfork)
 	jge	[blink]	; pid >=0 return, else detour via tailcall to errno
 
 	b   __syscall_error
-END(vfork)
+END(__vfork)
+
+weak_alias(__vfork,vfork)
 libc_hidden_def(vfork)