Provide a backward-compatible conditional define of O_CLOEXEC
for those systems which lack it, since these files are used in
tools builds.
- Modify message of PCIe capability version. This field (PCIE_XCAP_VER_MASK)
  is not specification's version number but the capability structure's version
  number. To avoid confusion, print "PCI Express capability version x".
- The max number of PCIe lane is not 16 but 32. Fix the bug using with macro.
- Use macro instead of magic number.
- Gb/s -> GT/s
Rename PCIE_XCAP_VER_* macros to avoid confusion.