selftests/powerpc/dexcr: Add DEXCR status utility lsdexcr

Add a utility 'lsdexcr' to print the current DEXCR status. Useful for
quickly checking the status such as when debugging test failures or
verifying the new default DEXCR does what you want (for userspace at
least). Example output:

    # ./lsdexcr
       uDEXCR: 04000000 (NPHIE)
       HDEXCR: 00000000
    Effective: 04000000 (NPHIE)

            SBHE   (0): clear  	(Speculative branch hint enable)
          IBRTPD   (3): clear  	(Indirect branch recurrent target ...)
           SRAPD   (4): clear  	(Subroutine return address ...)
           NPHIE * (5): set  	(Non-privileged hash instruction enable)
            PHIE   (6): clear  	(Privileged hash instruction enable)

    DEXCR[NPHIE] enabled: hashst/hashchk working

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230616034846.311705-12-bgray@linux.ibm.com