PowerPC64: Fix incorrect CFI in *context routines

PowerPC64: Fix incorrect CFI in *context routines

The context established by "makecontext" has a link register pointing
back to an error path within the makecontext routine.  This is currently
covered by the CFI FDE for makecontext itself, which is simply wrong
for the stack frame *inside* the context.  When trying to unwind (e.g.
doing a backtrace) in a routine inside a context created by makecontext,
this can lead to uninitialized stack slots being accessed, causing the
unwinder to crash in the worst case.

Similarly, during parts of the "setcontext" routine, when the stack
pointer has already been switched to point to the new context, the
address range is still covered by the CFI FDE for setcontext.  When
trying to unwind in that situation (e.g. backtrace from an async
signal handler for profiling), it is again possible that the unwinder
crashes.

Theses are all problems in existing code, but the changes in stack
frame layout appear to make the "worst case" much more likely in
the ELFv2 ABI context.  This causes regressions e.g. in the libgo
testsuite on ELFv2.

This patch fixes this by ending the makecontext/setcontext FDEs
before those problematic parts of the assembler, similar to what
is already done on other platforms.   This fixes the libgo
regression on ELFv2.