[libc][bazel] mark epoll funcitons as weak (llvm#81327)

Downstream there's a user that intercepts these functions and overlays
them. This causes symbol conflicts if neither function is marked weak.
In future the intent is to move to this to being a downstream
configuration
option.

Author: michaelrj-google

utils/bazel/llvm-project-overlay/libc/BUILD.bazel

@@ -3218,6 +3218,7 @@ libc_function(
     name = "epoll_wait",
     srcs = ["src/sys/epoll/linux/epoll_wait.cpp"],
     hdrs = ["src/sys/epoll/epoll_wait.h"],
+    weak = True,
     deps = [
         ":__support_osutil_syscall",
         ":errno",
@@ -3228,6 +3229,7 @@ libc_function(
     name = "epoll_pwait",
     srcs = ["src/sys/epoll/linux/epoll_pwait.cpp"],
     hdrs = ["src/sys/epoll/epoll_pwait.h"],
+    weak = True,
     deps = [
         ":__support_osutil_syscall",
         ":errno",
@@ -3240,6 +3242,7 @@ libc_function(
 #     name = "epoll_pwait2",
 #     srcs = ["src/sys/epoll/linux/epoll_pwait2.cpp"],
 #     hdrs = ["src/sys/epoll/epoll_pwait2.h"],
+#     weak = True,
 #     deps = [
 #         ":__support_osutil_syscall",
 #         ":errno",