Fix bug in nios2 prologue analysis.

Fix bug in nios2 prologue analysis.

The nios2 prologue analyzer was mistakenly using an unsigned int field
to represent a 32-bit signed value.  This caused problems with an
incorrect conversion being applied to negative values when they were
automatically promoted for addition to a 64-bit CORE_ADDR value.

This patch fixes test failures in gdb.base/large-frame.exp and
gdb.dwarf2/dw2-ref-missing-frame.exp.  Normally the nios2 backend
prefers to use the dwarf2 unwinder so the prologue analyzer is only
invoked if there is no dwarf2 information.

2019-08-23  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/
	* nios2-tdep.c (struct reg_value): Improve comments.  Make
	the offset field signed.