syscall.2, syscalls.2, getauxval.3: Add references to new vdso(7) page

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/syscall.2

@@ -145,7 +145,8 @@ The details for various architectures are listed in the two tables below.
 
 The first table lists the instruction used to transition to kernel mode,
 (which might not be the fastest or best way to transition to the kernel,
-so you might have to refer to the VDSO),
+so you might have to refer to
+.BR vdso (7)),
 the register used to indicate the system call number,
 and the register used to return the system call result.
 .if t \{\
@@ -219,4 +220,5 @@ main(int argc, char *argv[])
 .SH SEE ALSO
 .BR _syscall (2),
 .BR intro (2),
-.BR syscalls (2)
+.BR syscalls (2),
+.BR vdso (7)

man2/syscalls.2

@@ -833,4 +833,5 @@ and similarly
 .SH SEE ALSO
 .BR syscall (2),
 .BR unimplemented (2),
-.BR libc (7)
+.BR libc (7),
+.BR vdso (7)

man3/getauxval.3

@@ -210,7 +210,5 @@ see
 for more information.
 .SH SEE ALSO
 .BR secure_getenv (3),
+.BR vdso (7),
 .BR ld-linux.so (8)
-
-The kernel source file
-.IR Documentation/ABI/stable/vdso