Because missing registers are discarded, the register number returned by
GDB (when values are obtained) are not the same as the indices into the
internal list of registers. Explicitly store this mapping now.

Task-number: QTCREATORBUG-7468
Change-Id: Ica6fc6e6bd0a240cbd59923e4cabc7127e1aad8b
Reviewed-by: hjk <qthjk@ovi.com>