[r17-916 Regression] FAIL: decimal/make-decimal.cc -std=gnu++20 (test for excess errors) on Linux/x86_64

On Linux/x86_64,

f6971e28e71eda7f8a1d1247f57ee7f16828864d is the first bad commit
commit f6971e28e71eda7f8a1d1247f57ee7f16828864d
Author: Roger Sayle <[email protected]>
Date:   Thu May 28 20:50:11 2026 +0100

    x86_64 SSE: Handle SUBREG conversions in TImode STV (for ptest).

caused

FAIL: decimal/make-decimal.cc  -std=gnu++20 (test for excess errors)

with GCC configured with

../../gcc/configure --prefix=/export/users3/haochenj/src/gcc-bisect/master/master/r17-916/usr --enable-clocale=gnu --with-system-zlib --with-demangler-in-ld --with-fpmath=sse --enable-languages=c,c++,fortran --enable-cet --without-isl --enable-libmpx x86_64-linux --disable-bootstrap

To reproduce:

$ cd {build_dir}/x86_64-linux/libstdc++-v3/testsuite && make check RUNTESTFLAGS="conformance.exp=decimal/make-decimal.cc --target_board='unix{-m32\ -march=cascadelake}'"

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