android/build.py: link ARMv7 binary with libunwind

Fixes nullptr dereference when an exception gets thrown because there
is no ".eh_frame" section for unwinding.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/543

NEWS

@@ -1,4 +1,6 @@
 ver 0.21.9 (not yet released)
+* Android
+  - fix crash on ARMv7
 
 ver 0.21.8 (2019/04/23)
 * input

android/build.py

@@ -138,6 +138,12 @@ class AndroidNdkToolchain:
         libstdcxx_ldflags = libstdcxx_flags + ' -L' + libcxx_libs_path
         libstdcxx_libs = '-lc++_static -lc++abi'
 
+        if self.is_armv7:
+            # On 32 bit ARM, clang generates no ".eh_frame" section;
+            # instead, the LLVM unwinder library is used for unwinding
+            # the stack after a C++ exception was thrown
+            libstdcxx_libs += ' -lunwind'
+
         if use_cxx:
             self.cxxflags += ' ' + libstdcxx_cxxflags
             self.ldflags += ' ' + libstdcxx_ldflags