[libunwind] fixing semantics of UNW_IA64_PR

[David Mosberger]

Yesterday I noticed that access to the rotating portion (p16-p63) of
UNW_IA64_PR was busted.  In fact, there was some confusion in
libunwind whether or not bits 16-63 in this register should access the
rotated predicates.  The answer is that it should NOT and I fixed the
code to do just that.  I also added a test-case to check for this and
clarified the man-page libunwind-ia64(3).

For details, see:

 http://unwind.bkbits.net:8080/libunwind-v0.98/address@hidden

If you have software that accesses UNW_IA64_PR, please check it to
verify that it does NOT assume that bits 16-63 correspond to the
rotated predicate registers.  If it does, please change it so it uses
the following bit number to access predicate pN for N>=16:

 16 + (((N-16) + CFM_rrb_pr) % 48)

where CFM_rrb_pr can be obtained with:

        unw_word_t cfm, CFM_rrb_pr;

        unw_get_reg (&cursor, UNW_IA64_CFM, &cfm);
        CFM_rrb_pr = (cfm >> 32) & 0x3f;

For example, with CFM_rrb_pr == 3, p16 would correspond to bit 19 in
UNW_IA64_PR.

Since the access to PR.rot was busted anyway, I do not expect this fix
to cause serious issues, but if something crops up unexpectedly,
please let me know asap.

Thanks,

        --david