xtensa windowed: an aggregate result past 128 bytes of frame does not compile

Found on the S3: import re in any Nil Python program failed with target xtensa windowed: aggregate-result frame offset out of range. The reduced repro is a function that returns record ok: Boolean; n: Integer; a, b: array[0..13] of Integer end (120 B) through a local of that type. With 104 B it compiled. riscv32, x86-64 and xtensa Call0 were fine.

Mechanism

In windowed mode the hidden result pointer arrives in a2 and is stored to the result local's frame slot, addressed from a15, before the parameter copy. The address was formed with addi a9, a15, off, which takes -128..127, and any larger offset hit Error. The Call0 arm uses EmitFrameAddrXtensa, which materialises large offsets, but that helper bases on a7. In windowed mode, a7 is not the frame pointer yet at this point.

Fix

symtab.inc: past imm8, the offset goes through EmitLoadConstXtensa (movi, or a literal-pool l32r past ±2047) and add a9, a15, a9. That writes only a9, so a2 stays intact.

Test

test-xtensa, hosted (--platform=posix --xtensa-soft-mulhigh), with x86-64 as the oracle:

• test/test_xtensa_windowed_large_aggregate_result.pas at 120 B, 1 KB and 8 KB, under windowed and Call0;
• test/test_xtensa_windowed_regex_search.pas, which runs regex.pas (ReRunAt);
• a build-only row for Nil Python import re with the examples/esp32/nilpy-s3/build.sh flags.

On the compiler one commit back, the windowed builds fail.