runtime: use unsigned load for iscgo variable in ppc64x runtime asm

This changes the runtime asm code that loads iscgo to use MOVBZ
instead of MOVB, avoiding an unnecessary sign extension. This is most
significant in runtime.save_g, reducing the size from 8 to 7
instructions.

Change-Id: Iaa2121464b5309e1f27fd91b19b5603c7aaf619d
Reviewed-on: https://go-review.googlesource.com/c/144217
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/runtime/asm_ppc64x.s

@@ -648,7 +648,7 @@ TEXT ·cgocallback_gofunc(SB),NOSPLIT,$16-32
 	NO_LOCAL_POINTERS
 
 	// Load m and g from thread-local storage.
-	MOVB	runtime·iscgo(SB), R3
+	MOVBZ	runtime·iscgo(SB), R3
 	CMP	R3, $0
 	BEQ	nocgo
 	BL	runtime·load_g(SB)

src/runtime/sys_linux_ppc64x.s

@@ -301,7 +301,7 @@ TEXT runtime·_sigtramp(SB),NOSPLIT,$64
 
 	// this might be called in external code context,
 	// where g is not set.
-	MOVB	runtime·iscgo(SB), R6
+	MOVBZ	runtime·iscgo(SB), R6
 	CMP 	R6, $0
 	BEQ	2(PC)
 	BL	runtime·load_g(SB)

src/runtime/tls_ppc64x.s

@@ -24,7 +24,7 @@
 // NOTE: setg_gcc<> assume this clobbers only R31.
 TEXT runtime·save_g(SB),NOSPLIT|NOFRAME,$0-0
 #ifndef GOOS_aix
-	MOVB	runtime·iscgo(SB), R31
+	MOVBZ	runtime·iscgo(SB), R31
 	CMP	R31, $0
 	BEQ	nocgo
 #endif