[libunwind][Hexagon] Save/restore full r0-r7 in getcontext/jumpto (#208753)

The Hexagon save and restore routines dropped GPRs, producing an
incomplete context.

UnwindRegistersSave.S (__unw_getcontext): save r0-r7 (stores began at
__r[8]), drop a dead "r1 = memw(r30)" before the PC store, and return
UNW_ESUCCESS instead of a stale non-zero value.

UnwindRegistersRestore.S (jumpto): restore the missing r2-r7 (r0/r1 and
r8-r31 were already handled), and fix the stale "pointer is in r2"
comment (it is r0).

Signed-off-by: Kushal Pal <kushpal@qti.qualcomm.com>
GitOrigin-RevId: 7a8b26271b273ea36c1d68c97f4c87b63de4a2bd