[mips][codegen] Change two class's visibility to make cctest build pass

Building cctest has linking error when gn's args has "is_component_build = true".
This CL make the visibility of "MemOperand" and "UseScratchRegisterScope" from
"hidden" to "default" to solve this problem.

Change-Id: I3ebfaa25d598b4f2d626861b947c93d111d95456
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1731213
Auto-Submit: Mu Tao <pamilty@gmail.com>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63062}

src/codegen/mips/assembler-mips.h

@@ -122,7 +122,7 @@ class Operand {
 
 // On MIPS we have only one addressing mode with base_reg + offset.
 // Class MemOperand represents a memory operand in load and store instructions.
-class MemOperand : public Operand {
+class V8_EXPORT_PRIVATE MemOperand : public Operand {
  public:
   // Immediate value attached to offset.
   enum OffsetAddend { offset_minus_one = -1, offset_zero = 0 };
@@ -1872,7 +1872,7 @@ class EnsureSpace {
   explicit inline EnsureSpace(Assembler* assembler);
 };
 
-class UseScratchRegisterScope {
+class V8_EXPORT_PRIVATE UseScratchRegisterScope {
  public:
   explicit UseScratchRegisterScope(Assembler* assembler);
   ~UseScratchRegisterScope();

src/codegen/mips64/assembler-mips64.h

@@ -121,7 +121,7 @@ class Operand {
 
 // On MIPS we have only one addressing mode with base_reg + offset.
 // Class MemOperand represents a memory operand in load and store instructions.
-class MemOperand : public Operand {
+class V8_EXPORT_PRIVATE  MemOperand : public Operand {
  public:
   // Immediate value attached to offset.
   enum OffsetAddend { offset_minus_one = -1, offset_zero = 0 };
@@ -1899,7 +1899,7 @@ class EnsureSpace {
   explicit inline EnsureSpace(Assembler* assembler);
 };
 
-class UseScratchRegisterScope {
+class V8_EXPORT_PRIVATE UseScratchRegisterScope {
  public:
   explicit UseScratchRegisterScope(Assembler* assembler);
   ~UseScratchRegisterScope();