[libc] Add redirecting headers sys/vfs.h and sys/syslog.h on Linux. (#219118)

* `<sys/vfs.h>` is a Linux header for providing `statfs` function, and
can be used interchangeably with `<sys/statfs.h>`
(https://man7.org/linux/man-pages/man2/statfs.2.html). Simply redirect
the former to the latter, as `statfs` is already implemented.
* `<sys/syslog.h>` is frequently used instead of POSIX-specified
`<syslog.h>` header, so create the redirecting alias for it as well.

This would fix one of the problems in issue #97191 (building Clang
against LLVM-libc), since `<sys/vfs.h>` is actually used by LLVM
Support.

GitOrigin-RevId: 9a77747615e656f61f4389ef0f663f0f0b6e798b