runtime: save/restore g unconditionally on darwin/arm64

Now that we always have TLS set up, we can always save the G
register, regardless of whether cgo is used. This makes pure Go
programs signal-safe.

Updates #38485.

Change-Id: Icbc69acf0e2a5652fbcbbd074258a1a5efe87f1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/265119
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>

src/runtime/asm_arm64.s

@@ -977,10 +977,7 @@ TEXT ·cgocallback(SB),NOSPLIT,$24-24
 	NO_LOCAL_POINTERS
 
 	// Load g from thread-local storage.
-	MOVB	runtime·iscgo(SB), R3
-	CBZ	R3, nocgo
 	BL	runtime·load_g(SB)
-nocgo:
 
 	// If g is nil, Go did not create the current thread.
 	// Call needm to obtain one for temporary use.

src/runtime/sys_darwin_arm64.s

@@ -197,9 +197,6 @@ TEXT runtime·sigtramp(SB),NOSPLIT,$192
 
 	// this might be called in external code context,
 	// where g is not set.
-	MOVB	runtime·iscgo(SB), R0
-	CMP	$0, R0
-	BEQ	2(PC)
 	BL	runtime·load_g(SB)
 
 #ifdef GOOS_ios
@@ -381,7 +378,8 @@ TEXT runtime·mstart_stub(SB),NOSPLIT,$160
 	FMOVD	F14, 144(RSP)
 	FMOVD	F15, 152(RSP)
 
-	MOVD    m_g0(R0), g
+	MOVD	m_g0(R0), g
+	BL	·save_g(SB)
 
 	BL	runtime·mstart(SB)
 

src/runtime/tls_arm64.s

@@ -9,8 +9,10 @@
 #include "tls_arm64.h"
 
 TEXT runtime·load_g(SB),NOSPLIT,$0
+#ifndef TLS_darwin
 	MOVB	runtime·iscgo(SB), R0
 	CBZ	R0, nocgo
+#endif
 
 	MRS_TPIDR_R0
 #ifdef TLS_darwin
@@ -25,8 +27,10 @@ nocgo:
 	RET
 
 TEXT runtime·save_g(SB),NOSPLIT,$0
+#ifndef TLS_darwin
 	MOVB	runtime·iscgo(SB), R0
 	CBZ	R0, nocgo
+#endif
 
 	MRS_TPIDR_R0
 #ifdef TLS_darwin