[libc][errno] Deprecate `LIBC_ERRNO_MODE_SYSTEM` in favor of `LIBC_ERRNO_MODE_SYSTEM_INLINE`.

[lntue]

In LIBC_ERRNO_MODE_SYSTEM, we still create a temporary global object libc_errno and then point it to the system libc errno. To use system libc errno, it would be more efficient to make libc_errno a macro defined as errno, just as LIBC_ERRNO_MODE_SYSTEM_INLINE. It also allow us to skip linking against libc.src.errno.errno target.

[llvmbot]

@llvm/issue-subscribers-libc

Author: None (lntue)

In `LIBC_ERRNO_MODE_SYSTEM`, we still create a temporary global object `libc_errno` and then point it to the system libc `errno`. To use system libc `errno`, it would be more efficient to make `libc_errno` a macro defined as `errno`, just as `LIBC_ERRNO_MODE_SYSTEM_INLINE`. It also allow us to skip linking against `libc.src.errno.errno` target.