[codegen] Reduce assembler gap for arm64 and mips64

The gap is chosen unneccessarily large, leading to too early growing of
the assembler buffer.

R=mstarzinger@chromium.org

Change-Id: I9ddbe1d59929f2a76511cbc2d18c054fb8eafa74
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1914213
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64939}

src/codegen/arm64/assembler-arm64.h

@@ -2638,7 +2638,7 @@ class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
   // not have to check for overflow. The same is true for writes of large
   // relocation info entries, and debug strings encoded in the instruction
   // stream.
-  static constexpr int kGap = 128;
+  static constexpr int kGap = 64;
 
  public:
 #ifdef DEBUG

src/codegen/mips64/assembler-mips64.h

@@ -1642,7 +1642,7 @@ class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
   // the generated instructions. This is so that multi-instruction sequences do
   // not have to check for overflow. The same is true for writes of large
   // relocation info entries.
-  static constexpr int kGap = 128;
+  static constexpr int kGap = 64;
 
   // Repeated checking whether the trampoline pool should be emitted is rather
   // expensive. By default we only check again once a number of instructions