gn build: Fix Android build.

Remove --unwindlib=none flag that was added by D143598. It is required
to not pass this flag when linking binaries. With this flag, linking
e.g. llvm-symbolizer will fail.

There was a missing dependency from most targets to the implicitly linked
libraries (libunwind and builtins), which was causing the build for those
targets to fail when they were built on their own. I never noticed this
because I always build the implicitly linked libraries together with
their dependencies. Make the dependency explicit by introducing a new
target //llvm/utils/gn/build/libs/implicit representing the platform's
implicitly linked libraries, and depend on that from the LLVM libraries,
so that "normal" binaries like llvm-symbolizer will have all of their
dependencies available.

D143598 set the arch subdirectory to i686 for Android. However, the arch
subdirectory on Android is i386, and has been for a long time.

$ find android-ndk-r* -name i386
android-ndk-r18b/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/7.0.2/lib/linux/i386
android-ndk-r19/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/8.0.2/lib/linux/i386
android-ndk-r20/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/8.0.7/lib/linux/i386
android-ndk-r21-beta1/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/9.0.8/lib/linux/i386
android-ndk-r21d/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/9.0.8/lib/linux/i386
android-ndk-r22/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/11.0.5/lib/linux/i386
android-ndk-r23-beta5/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/12.0.5/lib/linux/i386
android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/14.0.1/lib/linux/i386
android-ndk-r25c/toolchains/llvm/prebuilt/linux-x86_64/lib64/clang/14.0.7/lib/linux/i386

Using the wrong name prevents Clang from being able to find libunwind.a on
x86. Fix the code to use i386 as the arch subdirectory, consistent with
the NDK.

Bring back -Wl,-z,defs which was removed for Android by D143598, presumably
because of the missing unwind library.

Differential Revision: https://reviews.llvm.org/D146266