[rfa/6.0] Better handle unspecified CFI values

[Andrew Cagney]

Hello,

This patch is an attempt at improving GDB's behavior when GCC "treads 
the boundaries of the CFI specification".

It does the following:

- changes the rules REG_UNMODIFIED -> REG_SAME_VALUE and REG_UNSAVED -> 
REG_UNDEFINED so that they better match the corresponding CFI register 
states (I could commit this separatly).  The other names confused me :-)

- it adds a new register rule - REG_UNSPECIFIED - which is used to 
differentiate a register that is missing CFI info from a register that 
CFI specified as "undefined" (nee UNSAVED).

- when unwinding, it treats REG_UNSPECIFIED registers like 
REG_SAME_VALUE but with the additional hack to map an unspecified 
SP_REGNUM onto the CFA.

- if it detects an unspecified CFI entry it complains
It isn't perfect though - since it doesn't know the full range of valid 
debug info register numbers it can't check every entry.  Instead it 
checks the range provided by CFI for unspecified holes and then 
complains about that.  The reality is that GCC at least gets that bit 
right (but consistently forgets the SP).

I'd like to commit the patch as is for the 6.0 branch.  For the mainline 
though, I'd like to make the additional changes:

- delete the SP_REGNUM hack from the REG_UNDEFINED rule (it's no longer 
needed, I think)

- add a check/complaint for the SP v CFA problem.

Anyway, the end result is that on x86-64 and i386 store.exp now passes.

ok to commit?
Andrew
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