[libc] disable epoll_pwait2 due to breakage (#80051)

It appears that sys_epoll_pwait2 isn't always available, so we need to
add some sort of condition to enable it. This patch disables it until
that happens.

Author: michaelrj-google

libc/config/linux/x86_64/entrypoints.txt

@@ -146,7 +146,9 @@ set(TARGET_LIBC_ENTRYPOINTS
     # sys/epoll.h entrypoints
     libc.src.sys.epoll.epoll_wait
     libc.src.sys.epoll.epoll_pwait
-    libc.src.sys.epoll.epoll_pwait2
+    # TODO: Need to check if pwait2 is available before providing.
+    # https://github.com/llvm/llvm-project/issues/80060
+    # libc.src.sys.epoll.epoll_pwait2
 
     # sys/mman.h entrypoints
     libc.src.sys.mman.madvise