I ran into the problem again. i tried to make version 5.5.4 and error
__sync_fetch_and_add_8
sys/libmysys.a ../../dbug/libdbug.a ../../strings/libmystrings.a -lpthread -lm -lpthread
/home/admin/jj/staging_dir/toolchain-mipsel_24kc_gcc-7.5.0_musl/bin/../lib/gcc/mipsel-openwrt-linux-musl/7.5.0/../../../../mipsel-openwrt-linux-musl/bin/ld: my_atomic-t.o: in function `test_atomic_add64':
my_atomic-t.c:(.text+0x368): undefined reference to `__sync_fetch_and_add_8'
/home/admin/jj/staging_dir/toolchain-mipsel_24kc_gcc-7.5.0_musl/bin/../lib/gcc/mipsel-openwrt-linux-musl/7.5.0/../../../../mipsel-openwrt-linux-musl/bin/ld: my_atomic-t.c:(.text+0x38c): undefined reference to `__sync_fetch_and_add_8'
/home/admin/jj/staging_dir/toolchain-mipsel_24kc_gcc-7.5.0_musl/bin/../lib/gcc/mipsel-openwrt-linux-musl/7.5.0/../../../../mipsel-openwrt-linux-musl/bin/ld: my_atomic-t.c:(.text+0x3a0): undefined reference to `__sync_fetch_and_add_8'
/home/admin/jj/staging_dir/toolchain-mipsel_24kc_gcc-7.5.0_musl/bin/../lib/gcc/mipsel-openwrt-linux-musl/7.5.0/../../../../mipsel-openwrt-linux-musl/bin/ld: my_atomic-t.c:(.text+0x3ac): undefined reference to `__sync_fetch_and_add_8'
/home/admin/jj/staging_dir/toolchain-mipsel_24kc_gcc-7.5.0_musl/bin/../lib/gcc/mipsel-openwrt-linux-musl/7.5.0/../../../../mipsel-openwrt-linux-musl/bin/ld: my_atomic-t.o: in function `do_tests':
my_atomic-t.c:(.text+0x7e0): undefined reference to `__sync_fetch_and_add_8'
/home/admin/jj/staging_dir/toolchain-mipsel_24kc_gcc-7.5.0_musl/bin/../lib/gcc/mipsel-openwrt-linux-musl/7.5.0/../../../../mipsel-openwrt-linux-musl/bin/ld: my_atomic-t.o:my_atomic-t.c:(.text+0x7f4): more undefined references to `__sync_fetch_and_add_8' follow
collect2: error: ld returned 1 exit status
Makefile:627: recipe for target 'my_atomic-t' failed
make[5]: *** [my_atomic-t] Error 1
make[5]: Leaving directory '/home/admin/jj/build_dir/target-mipsel_24kc_musl/mysql-5.5.4-m3/unittest/mysys'
Makefile:570: recipe for target 'all-recursive' failed