Fix getting tunable values on big-endian (BZ #21109)

Fix getting tunable values on big-endian (BZ #21109)

The code to set value passed a tunable_val_t, which when cast to
int32_t on big-endian gives the wrong value.  Instead, use
tunable_val_t.numval instead, which can then be safely cast into
int32_t.

(cherry picked from commit 8cbc826c37c0221ada65a7a622fe079b4e89a4b0)
(cherry picked from commit 58520986c38e34db60e07260c64c563e3efcf353)