S390: Recognize special jumps in prologue parser

S390: Recognize special jumps in prologue parser

Functions compiled with the gcc option `-mhotpatch' may start with a
branch-never BRCL instruction as a 6-byte NOP.  And functions compiled
with `-mstack-size' contain a BRC instruction in their prologue that is
actually a conditional trap.  Both of these special jumps cause the
prologue parser to stop and yield bad unwinding results.

This change makes the prologue analyzer recognize such special jumps and
ignore them.

gdb/ChangeLog:

	* s390-linux-tdep.c (s390_analyze_prologue): Ignore BRC and BRCL
	instructions that do nothing or are conditional traps.