selftests: vDSO: vdso_standalone_test_x86: Replace source file with symlink

With the switch over to nolibc the source file vdso_standalone_test_x86.c
was intended to be replaced with a symlink to vdso_test_gettimeofday.c.
This was the patch that was submitted to LKML, but during application the
symlink was replaced by a textual copy of the linked-to file.

Having two copies introduces the possibility of divergence and increases
maintenance burden, switch back to a symlink.

Fixes: 8770a91 ("selftests: vDSO: vdso_standalone_test_x86: Switch to nolibc")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/20250226-parse_vdso-nolibc-v2-16-28e14e031ed8@linutronix.de/
Link: https://lore.kernel.org/all/20250611-selftests-vdso-fixes-v3-9-e62e37a6bcf5@linutronix.de

Author: t-8ch

tools/testing/selftests/vDSO/vdso_standalone_test_x86.c

@@ -0,0 +1 @@
+vdso_test_gettimeofday.c