[PATCH 0/1] MR45: ntdll: Always prefer native signal restorer over the custom one.

[Jinoh Kang (＠iamahuman)]

A few months after commit 1b1ea639cd6 (ntdll: Use a custom signal
restorer for Android., 2014-07-17) has been introduced, bionic rolled
its own signal restorer.  The patch is at:

https://android-review.googlesource.com/c/platform/bionic/+/107692

In general, rolling our own signal restorer is not a good idea since it
interferes with unwinding.  GDB seems especially unhappy when the signal
restorer's name isn't exactly "__restore_rt".

Fix this by using the custom signal restorer only if the vDSO has
disappeared and libc doesn't supply its own signal restorer.  This also
removes Android-specific code, although there aren't many libcs out
there that lack a backup signal restorer.

Signed-off-by: Jinoh Kang