[libc] Do not include system header `errno.h` in `libc_errno.h` header.

[lntue]

Separate just the definition of errno numbers in include/llvm-libc-macros/* and only include that instead of the system header in libc_errno.h

[nickdesaulniers]

Looking at the sources of the linux kernel, FWICT, the values here don't appear to be architecturally dependent.

That said, it looks like glibc may have different values for

• ECANCELED
• EOWNERDEAD
• ENOTRECOVERABLE
• ERFKILL
• EHWPOISON
  for:
• mips
• alpha
• sparc
• hppa (parisc?)

(Hurd also has significantly different values, so perhaps the above only applies for linux).

(LLVM itself only supports mips and sparc (poorly) from the above list) cc @frobtech

@frobtech does Fuchsia use values for the various E* macros defined by errno.h that differ from Linux?

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Right now ninja libc-lint is spewing instances of llvmlibc-restrict-system-libc-headers over including <errno.h> in libc/src/errno/libc_errno.h, but I suspect we need some way of having a different set of values for errno (if fuchsia has different values than Linux).