Triage & fix new GCC test failures before 15.1.0 release

[xry111]

• FAIL: c-c++-common/toplevel-asm-1.c -Wc++-compat (test for excess errors)
  • This is an easy hack. Just append || defined (__loongarch__) after riscv and it'll be fine.
  • https://gcc.gnu.org/pipermail/gcc-patches/2025-February/675106.html
• FAIL: gcc.dg/pr108692.c execution test https://gcc.gnu.org/PR118727
  • https://gcc.gnu.org/r15-7370
• FAIL: gcc.dg/torture/pr113026-2.c -O3 -g (test for bogus messages, line 17, line 9) https://gcc.gnu.org/PR118504#c1
• FAIL: gcc.dg/tree-ssa/gen-vect-*.c maybe https://gcc.gnu.org/PR113418
• FAIL: gcc.dg/tree-ssa/predcom-8.c scan-tree-dump-not pcom "Invalid sum" https://gcc.gnu.org/PR118407
• FAIL: gcc.dg/vect/bb-slp-77.c scan-tree-dump-times slp1 "optimized: basic block" 1
• FAIL: gcc.dg/vect/slp-26.c scan-tree-dump-times vect "vectorizing stmts using SLP" 1
• FAIL: gcc.dg/vect/vect-121.c scan-tree-dump-not optimized "Invalid sum"
• FAIL: gcc.dg/vect/vect-epilogues.c scan-tree-dump-not optimized "Invalid sum"
• FAIL: gcc.dg/vect/vect-reduc-chain-2.c scan-tree-dump vect "vectorizing statement: \\S+ = DOT_PROD_EXPR"
  • https://gcc.gnu.org/pipermail/gcc-patches/2025-February/675294.html
• FAIL: gcc.dg/vect/vect-reduc-chain-1.c scan-tree-dump-times vect "vectorizing statement: \\S+ = DOT_PROD_EXPR" 2
  • The patch for previous one seems not enough
• FAIL: gcc.dg/vect/vect-rounding-*.c scan-tree-dump-times vect "vectorized 1 loops" 1
  • https://gcc.gnu.org/pipermail/gcc-patches/2025-February/675292.html
  • https://gcc.gnu.org/r15-7465

[xry111]

For vect-rounding-*.c the problem is GCC 15 now defaults to -std=gnu23, where vfrint instructions cannot be used anymore as they may raise INEXACT exceptions those C23 disallows for ceil, floor, and etc.