Add MIPS support to GetProgramCounter()

Change adds __mips__ case to the GetProgramCounter function in abseil.

Chromium bug: 850029

absl/debugging/internal/examine_stack.cc

diff --git a/absl/debugging/internal/examine_stack.cc b/absl/debugging/internal/examine_stack.cc
index 261daae..8434709 100644
--- a/absl/debugging/internal/examine_stack.cc
+++ b/absl/debugging/internal/examine_stack.cc
@@ -46,6 +46,8 @@
 #elif defined(__i386__)
     if (14 < ABSL_ARRAYSIZE(context->uc_mcontext.gregs))
       return reinterpret_cast<void*>(context->uc_mcontext.gregs[14]);
+#elif defined(__mips__)
+    return reinterpret_cast<void*>(context->uc_mcontext.pc);
 #elif defined(__powerpc64__)
     return reinterpret_cast<void*>(context->uc_mcontext.gp_regs[32]);
 #elif defined(__powerpc__)