The ConstantRelocatable objects for pushing local labels are allocated from the Assembler arena, and are no longer pooled, which restricts the memory growth from sandboxing x86-64 calls.

Because the Assembler arena is destroyed while the fixups are still active, these fixups have to be fixed up by holding a pointer to the symbol rather than the constant.

On the 10MB test pexe, the overall growth by the end is ~20MB, instead of ~130MB as before.

This also partially fixes an existing bug with arm32/nonsfi/iasm, exposed by running cross tests and forcing iasm output.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1773503003 .