[libc] [llvm] [libc] Move libc_errno.h to libc/src/__support and make LIBC_ERRNO_MODE_SYSTEM to be header-only. (PR #143187)

[via llvm-commits]

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@@ -46,4 +46,7 @@ LIBC_CONFIGURE_OPTIONS = [
 
     # Documentation in libc/src/__support/libc_assert.h
     # "LIBC_COPT_USE_C_ASSERT",
+
+    # Use system errno.
+    "LIBC_ERRNO_MODE=LIBC_ERRNO_MODE_SYSTEM",
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lntue wrote:

Probably I should add another mode instead, to preserve the current behavior.  And we can test with the flag flip later.

https://github.com/llvm/llvm-project/pull/143187