AcpiDump: tidy up for FreeBSD

- Use AcpiTbValidateRsdp () from tbxfroot to reduce the code duplication.
- While I am here, sync. FreeBSD comments with Linux version.
- Move few platform-specific definitions from FreeBSD header files to platform/acfreebsd.h.
- Use "uname -p" instead of "uname -m" to detect 64-bit processors on FreeBSD.
- Fix binary installations for FreeBSD. Also, strip binaries and set mode.