Improve system call polyfills

- Polyfill open() w/ O_CLOEXEC on RHEL5
- Remove old workaround from rmdir() on the New Technology
- preadv() and pwritev() are now smarter about demodernization
- preadv() and pwritev() are now available on the New Technology

libc/sysv/calls/__sys_openat.s

@@ -0,0 +1,2 @@
+.include "o/libc/sysv/macros.internal.inc"
+.scall __sys_openat,0x1d41411f321cf101,globl,hidden

libc/sysv/calls/sys_openat.s

@@ -1,2 +0,0 @@
-.include "o/libc/sysv/macros.internal.inc"
-.scall sys_openat,0x1d41411f321cf101,globl,hidden