libsampler fix register calls for FreeBSD

Those fields are incorrect in FreeBSD.
Upstreaming local FreeBSD patches.

Change-Id: I28cf6dbec1d5e4d26e62dd9a0d78d039c3e36cdb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906374Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65226}

src/libsampler/sampler.cc

@@ -477,9 +477,9 @@ void SignalHandler::FillRegisterState(void* context, RegisterState* state) {
   state->sp = reinterpret_cast<void*>(mcontext.mc_rsp);
   state->fp = reinterpret_cast<void*>(mcontext.mc_rbp);
 #elif V8_HOST_ARCH_ARM
-  state->pc = reinterpret_cast<void*>(mcontext.mc_r15);
-  state->sp = reinterpret_cast<void*>(mcontext.mc_r13);
-  state->fp = reinterpret_cast<void*>(mcontext.mc_r11);
+  state->pc = reinterpret_cast<void*>(mcontext.__gregs[_REG_PC]);
+  state->sp = reinterpret_cast<void*>(mcontext.__gregs[_REG_SP]);
+  state->fp = reinterpret_cast<void*>(mcontext.__gregs[_REG_FP]);
 #endif  // V8_HOST_ARCH_*
 #elif V8_OS_NETBSD
 #if V8_HOST_ARCH_IA32