Replace libc's call to pipe(2) with pipe2(2).

[brooks]

Summary

pipe2() with flags=0 is idential to pipe(), but the pipe() system call
abuses the fact that system calls return two registers to avoid the use of
copyout() making it hard to trace with dtrace (since the kernel ignores
the arguments) and requring a machine dependent assembly wrapper in libc
for each version.

[andrew]

Does pipe.o need to be added to NOASM, or does t figure this out when pipe.c is in SRCS?

[brooks]

Add pipe.o to NOASM and fix a missing declaration.

[brooks]

It looks like NOASM wasn't required in my test tree because D6816 was applied. Now fixed.

[emaste]

Is this intended to be done for 11.0, or after?

[brooks]

In D6815#144317, @emaste wrote:

Is this intended to be done for 11.0, or after?

I'd like to squeeze it in for 11 so aarch64 and riscv can not support the syscall, but it's not critical. I need to test the result more thoroughly before I can propose it.