"undefined reference to `__builtin_bswap64'" on mips64el/loongson

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Hi OpenBSD ports maintainers,

I'm having trouble building security/libnettle on mips64el/loongson
which is caused by missing symbol of "__builtin_bswap64" when linking.
It looks like this symbol is introduced since GCC 4.3[1], while mips64el
ships with GCC 4.2.1. It's interesting because I can compile with the
symbol but cannot link. Would like to hear from the ports maintainers'
opinion on how to solve this issue?

Here are some tests for showing the problem (please let me know if you
need more information):

$ uname -a
OpenBSD yeeloong.lan 6.6 GENERIC#18 loongson
$ gcc -v
Reading specs from /usr/lib/gcc-lib/mips64el-unknown-openbsd6.6/4.2.1/specs
Target: mips64el-unknown-openbsd6.6
Configured with: OpenBSD/mips64el system compiler
Thread model: posix
gcc version 4.2.1 20070719
$ cat test.c
#include <stdint.h>

int
main ()
{

uint64_t x = 17;
uint64_t y = __builtin_bswap64(x);

;
return 0;
}
$ gcc -c test.c
$ gcc test.c
/tmp//ccw1b7KO.o: In function `main':
test.c:(.text+0x2c): undefined reference to `__builtin_bswap64'
test.c:(.text+0x30): undefined reference to `__builtin_bswap64'
collect2: ld returned 1 exit status

[1] http://gcc.gnu.org/onlinedocs/gcc-4.3.0/gcc/Other-Builtins.html