[PowerPC] Fix *context on ELFv2

Due to ELFv1 specific code in _ctx_start.S and makecontext.c, userspace
context switching was completely broken on ELFv2.

With this change, we now pass the libthr test suite.

Approved by: jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D22421

lib/libc/powerpc64/gen/_ctx_start.S

@@ -32,8 +32,14 @@
 	.globl  CNAME(abort)
 
  ENTRY(_ctx_start)
+#if !defined(_CALL_ELF) || _CALL_ELF == 1
+	/* Load values from function descriptor */
 	ld	%r2,8(%r14)
 	ld	%r14,0(%r14)
+#else
+	/* Load global entry point */
+	mr	%r12,%r14
+#endif
 	mtlr	%r14
 	blrl			/* branch to start function */
 	mr	%r3,%r15	/* pass pointer to ucontext as argument */

lib/libc/powerpc64/gen/makecontext.c

@@ -113,7 +113,12 @@ __makecontext(ucontext_t *ucp, void (*start)(void), int argc, ...)
 	 * Use caller-saved regs 14/15 to hold params that _ctx_start
 	 * will use to invoke the user-supplied func
 	 */
+#if !defined(_CALL_ELF) || _CALL_ELF == 1
+	/* Cast to ensure this is treated as a function descriptor. */
 	mc->mc_srr0 = *(uintptr_t *)_ctx_start;
+#else
+	mc->mc_srr0 = (uintptr_t) _ctx_start;
+#endif
 	mc->mc_gpr[1] = (uintptr_t) sp;		/* new stack pointer */
 	mc->mc_gpr[14] = (uintptr_t) start;	/* r14 <- start */
 	mc->mc_gpr[15] = (uintptr_t) ucp;	/* r15 <- ucp */