[MIPS] Add __popcountsi2 to libgcc_compat.c

Also removed outdated genlibgcc_compat.py script
and references to it.

Bug: http://b/27340856
Change-Id: I7eef362364078867c60a27e72abba515f08b6651
diff --git a/libc/arch-mips/mips.mk b/libc/arch-mips/mips.mk
index b184abb..3663fab 100644
--- a/libc/arch-mips/mips.mk
+++ b/libc/arch-mips/mips.mk
@@ -15,6 +15,7 @@
     arch-mips/bionic/bzero.S \
     arch-mips/bionic/cacheflush.cpp \
     arch-mips/bionic/_exit_with_stack_teardown.S \
+    arch-mips/bionic/libgcc_compat.c \
     arch-mips/bionic/setjmp.S \
     arch-mips/bionic/syscall.S \
     arch-mips/bionic/vfork.S \