Switch to -mfpu=vfpv3-d16 for ARM32 runtime builds

armeabi-v7a libs should not use registers d16 to d31

NDK r23c will be the last NDK to support users setting -mfpu=vfpv3-d16
manually, so we need to build our runtime libraries this way one last
time. Future toolchains and NDKs can build things as NEON (or more
specifically as the default armv7a Android configuration).

Bug: https://github.com/android/ndk/issues/1672
Test: ./build.py
Test: https://github.com/android/ndk/issues/1672#issuecomment-1051945802
Change-Id: I3bd752cfee799ef1f139c6c9afbe07cdffab746a
diff --git a/configs.py b/configs.py
index cff4799..de4c8de 100644
--- a/configs.py
+++ b/configs.py
@@ -349,6 +349,9 @@
     def cflags(self) -> List[str]:
         cflags = super().cflags
         cflags.append('-march=armv7-a')
+        # https://github.com/android/ndk/issues/1672
+        # armeabi-v7a libs should not use registers d16 to d31
+        cflags.append('-mfpu=vfpv3-d16')
         return cflags