tunnel: libwg-go: use deterministic verdef name

From the ld.gold source:

  const char* name = parameters->options().soname();
  if (name == NULL)
    name = parameters->options().output_file_name();

That means by default it uses `-o {output}` as the verdef aux name,
which is random every time due to Go's build system. By passing in
`--soname={somethingexplicit}` we can instead have a deterministic
verdef.

This commit makes wireguard-android reproducible.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tunnel/tools/libwg-go/Makefile

@@ -14,7 +14,7 @@ NDK_GO_ARCH_MAP_mips64 := mips64x
 
 CLANG_FLAGS := --target=$(ANDROID_LLVM_TRIPLE) --gcc-toolchain=$(ANDROID_TOOLCHAIN_ROOT) --sysroot=$(ANDROID_SYSROOT)
 export CGO_CFLAGS := $(CLANG_FLAGS) $(CFLAGS)
-export CGO_LDFLAGS := $(CLANG_FLAGS) $(LDFLAGS)
+export CGO_LDFLAGS := $(CLANG_FLAGS) $(LDFLAGS) -Wl,-soname=libwg-go.so
 export CC := $(ANDROID_C_COMPILER)
 export GOARCH := $(NDK_GO_ARCH_MAP_$(ANDROID_ARCH_NAME))
 export GOOS := android