[PPC64] Fix trapstk overflow

In some scenarios, the 4K trapstk may overflow, corrupting tmpstk.

This was observed during remote debugging, with the following steps:

At remote host (R):
- enter kdb during boot
- switch to gdb backend

At local host (L):
- attach gdb to R
- try to read an invalid memory position

At R:
- a DSI trap occurs and kdb restarts (all this occurs on trapstk)
- while printing the stacktrace, trapstk overflows and corrupts tmpstk

Reviewed by:	jhibbits
Differential Revision:	https://reviews.freebsd.org/D22200

sys/powerpc/aim/locore64.S

@@ -66,7 +66,7 @@ TOC_ENTRY(tmpstk)
 TOC_ENTRY(can_wakeup)
 
 #ifdef KDB
-#define TRAPSTKSZ       4096            /* 4k trap stack */
+#define TRAPSTKSZ       8192            /* 8k trap stack */
 GLOBAL(trapstk)
         .space        TRAPSTKSZ
 TOC_ENTRY(trapstk)