runtime: fix crash during VDSO calls on PowerPC

This patch reinstates a fix for PowerPC with regard to making VDSO calls
while receiving a signal, and subsequently crashing. The crash happens
because certain VDSO calls can modify the r30 register, which is where g
is stored. This change was reverted for PowerPC because r30 is supposed
to be a non-volatile register. This is true, but that only makes a
guarantee across function calls, but not "within" a function call. This
patch was seemingly fine before because the Linux kernel still had hand
rolled assembly VDSO function calls, however with a recent change to C
function calls it seems the compiler used can generate instructions
which temporarily clobber r30. This means that when we receive a signal
during one of these calls the value of r30 will not be the g as the
runtime expects, causing a segfault.

You can see from this assembly dump how the register is clobbered during
the call:

(the following is from a 5.13rc2 kernel)

```
Dump of assembler code for function __cvdso_clock_gettime_data:
   0x00007ffff7ff0700 <+0>:     cmplwi  r4,15
   0x00007ffff7ff0704 <+4>:     bgt     0x7ffff7ff07f0 <__cvdso_clock_gettime_data+240>
   0x00007ffff7ff0708 <+8>:     li      r9,1
   0x00007ffff7ff070c <+12>:    slw     r9,r9,r4
   0x00007ffff7ff0710 <+16>:    andi.   r10,r9,2179
   0x00007ffff7ff0714 <+20>:    beq     0x7ffff7ff0810 <__cvdso_clock_gettime_data+272>
   0x00007ffff7ff0718 <+24>:    rldicr  r10,r4,4,59
   0x00007ffff7ff071c <+28>:    lis     r9,32767
   0x00007ffff7ff0720 <+32>:    std     r30,-16(r1)
   0x00007ffff7ff0724 <+36>:    std     r31,-8(r1)
   0x00007ffff7ff0728 <+40>:    add     r6,r3,r10
   0x00007ffff7ff072c <+44>:    ori     r4,r9,65535
   0x00007ffff7ff0730 <+48>:    lwz     r8,0(r3)
   0x00007ffff7ff0734 <+52>:    andi.   r9,r8,1
   0x00007ffff7ff0738 <+56>:    bne     0x7ffff7ff07d0 <__cvdso_clock_gettime_data+208>
   0x00007ffff7ff073c <+60>:    lwsync
   0x00007ffff7ff0740 <+64>:    mftb    r30      <---- RIGHT HERE
=> 0x00007ffff7ff0744 <+68>:    ld      r12,40(r6)
```

What I believe is happening is that the kernel changed the PowerPC VDSO
calls to use standard C calls instead of using hand rolled assembly. The
hand rolled assembly calls never touched r30, so this change was safe to
roll back. That does not seem to be the case anymore as on the 5.13rc2
kernel the compiler *is* generating assembly which modifies r30, making
this change again unsafe and causing a crash when the program receives a
signal during these calls (which will happen often due to async
preempt).

I realize this was reverted due to unexplained hangs in PowerPC
builders, but I think we should reinstate this change and investigate
those issues separately:
f4ca3c1

Author: derekparker

src/runtime/signal_unix.go

@@ -382,7 +382,7 @@ func preemptM(mp *m) {
 //go:nosplit
 func sigFetchG(c *sigctxt) *g {
 	switch GOARCH {
-	case "arm", "arm64":
+	case "arm", "arm64", "ppc64", "ppc64le":
 		if !iscgo && inVDSOPage(c.sigpc()) {
 			// When using cgo, we save the g on TLS and load it from there
 			// in sigtramp. Just use that.