std.debug.StackIterator: account for SPARC %fp quirk

On SPARC, previous %fp is saved with a 14 slots offset from current %fp+bias.
Also account for the bias constant at the new_fp calculation.
1eb2e48014Koakuma committed on 2/4/2021, 1:51:41 PMparent2d447b5
1 file changedLine totals unavailable