[libc][bazel] mark read and write as weak (llvm#81330)

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

@@ -2803,6 +2803,7 @@ libc_function(
     name = "read",
     srcs = ["src/unistd/linux/read.cpp"],
     hdrs = ["src/unistd/read.h"],
+    weak = True,
     deps = [
         ":__support_common",
         ":__support_macros_sanitizer",
@@ -2928,6 +2929,7 @@ libc_function(
     name = "write",
     srcs = ["src/unistd/linux/write.cpp"],
     hdrs = ["src/unistd/write.h"],
+    weak = True,
     deps = [
         ":__support_common",
         ":__support_osutil_syscall",