sysutils/btop: fix build on i386 and powerpc with GCC 11

/usr/local/bin/ld: /tmp/cc4flGwQ.ltrans0.ltrans.o: in function `Runner::_runner(void*)':
<artificial>:(.text+0x3c74): undefined reference to `atomic_load_8'
/usr/local/bin/ld: <artificial>:(.text+0x3c9c): undefined reference to `atomic_load_8'
/usr/local/bin/ld: <artificial>:(.text+0x3cc8): undefined reference to `atomic_compare_exchange_8'
/usr/local/bin/ld: <artificial>:(.text+0x441c): undefined reference to `atomic_load_8'
/usr/local/bin/ld: <artificial>:(.text+0x4448): undefined reference to `atomic_compare_exchange_8'
/usr/local/bin/ld: <artificial>:(.text+0x446c): undefined reference to `atomic_load_8'
/usr/local/bin/ld: <artificial>:(.text+0x4480): undefined reference to `atomic_load_8'
/usr/local/bin/ld: <artificial>:(.text+0x44ac): undefined reference to `atomic_compare_exchange_8'
/usr/local/bin/ld: /tmp/cc4flGwQ.ltrans1.ltrans.o: in function `Runner::run(std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, bool, bool)':
<artificial>:(.text+0x1cf4): undefined reference to `atomic_fetch_add_8'
/usr/local/bin/ld: <artificial>:(.text+0x1d44): undefined reference to `atomic_fetch_add_8'
/usr/local/bin/ld: /tmp/cc4flGwQ.ltrans1.ltrans.o: in function `Runner::stop()':
<artificial>:(.text+0x5150): undefined reference to `atomic_fetch_add_8'
/usr/local/bin/ld: <artificial>:(.text+0x51a8): undefined reference to `__atomic_fetch_add_8'
collect2: error: ld returned 1 exit status