MIPS: Fix unittests compilation error due to missing class exports

Fix `d324382e`

Change-Id: I00d15869ab6ee150f458ecbc1688308ebd5a5819
Reviewed-on: https://chromium-review.googlesource.com/1150151Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ivica Bogosavljevic <ivica.bogosavljevic@mips.com>
Cr-Commit-Position: refs/heads/master@{#54698}

src/mips/assembler-mips.h

@@ -475,8 +475,7 @@ class MemOperand : public Operand {
   friend class Assembler;
 };
 
-
-class Assembler : public AssemblerBase {
+class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
  public:
   // Create an assembler. Instructions and relocation information are emitted
   // into a buffer, with the instructions starting from the beginning and the

src/mips/macro-assembler-mips.h

@@ -131,7 +131,7 @@ inline MemOperand CFunctionArgumentOperand(int index) {
   return MemOperand(sp, offset);
 }
 
-class TurboAssembler : public TurboAssemblerBase {
+class V8_EXPORT_PRIVATE TurboAssembler : public TurboAssemblerBase {
  public:
   TurboAssembler(Isolate* isolate, const AssemblerOptions& options,
                  void* buffer, int buffer_size,

src/mips64/assembler-mips64.h

@@ -482,8 +482,7 @@ class MemOperand : public Operand {
   friend class Assembler;
 };
 
-
-class Assembler : public AssemblerBase {
+class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
  public:
   // Create an assembler. Instructions and relocation information are emitted
   // into a buffer, with the instructions starting from the beginning and the

src/mips64/macro-assembler-mips64.h

@@ -148,7 +148,7 @@ inline MemOperand CFunctionArgumentOperand(int index) {
   return MemOperand(sp, offset);
 }
 
-class TurboAssembler : public TurboAssemblerBase {
+class V8_EXPORT_PRIVATE TurboAssembler : public TurboAssemblerBase {
  public:
   TurboAssembler(Isolate* isolate, const AssemblerOptions& options,
                  void* buffer, int buffer_size,