Fix build on PPC arch with musl

Some C libraries, notably Musl, define the regs member as a void pointer,
hence we need to use the gp_regs member instead.

Change-Id: I1ca2dbdba79a03ff81c25438c87c767d7a7cece4
Reviewed-on: https://chromium-review.googlesource.com/602327Reviewed-by: Jaideep Bajwa <bjaideep@ca.ibm.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47481}

src/libsampler/sampler.cc

@@ -450,11 +450,18 @@ void SignalHandler::FillRegisterState(void* context, RegisterState* state) {
   state->sp = reinterpret_cast<void*>(mcontext.gregs[29]);
   state->fp = reinterpret_cast<void*>(mcontext.gregs[30]);
 #elif V8_HOST_ARCH_PPC
+#if V8_LIBC_GLIBC
   state->pc = reinterpret_cast<void*>(ucontext->uc_mcontext.regs->nip);
   state->sp =
       reinterpret_cast<void*>(ucontext->uc_mcontext.regs->gpr[PT_R1]);
   state->fp =
       reinterpret_cast<void*>(ucontext->uc_mcontext.regs->gpr[PT_R31]);
+#else
+  // Some C libraries, notably Musl, define the regs member as a void pointer
+  state->pc = reinterpret_cast<void*>(ucontext->uc_mcontext.gp_regs[32]);
+  state->sp = reinterpret_cast<void*>(ucontext->uc_mcontext.gp_regs[1]);
+  state->fp = reinterpret_cast<void*>(ucontext->uc_mcontext.gp_regs[31]);
+#endif
 #elif V8_HOST_ARCH_S390
 #if V8_TARGET_ARCH_32_BIT
   // 31-bit target will have bit 0 (MSB) of the PSW set to denote addressing