Add __popcountsi2 to the function compat list.

This is needed if we use Clang to compile Bionic, which won't include
__popcountsi2 anymore as Clang generates inline instructions. However
prebuilt binary blobs still depend on libc.so to resolve __popcountsi2.

Change-Id: I9001a3884c4be250c0ceebcd79922783fae1a0b7

libc/arch-arm/bionic/libgcc_compat.c

@@ -152,6 +152,7 @@
     XX(__muldi3)             \
     XX(__mulsf3)             \
     XX(__nedf2)              \
+    XX(__popcountsi2)        \
     XX(__subdf3)             \
     XX(__subsf3)             \
     XX(__truncdfsf2)         \