cgen: include stdio for GCC MinGW prelude

[medvednikov]

Fixes #27512.

Summary

• Include <stdarg.h> and <stdio.h> for the GCC+MinGW generated C prelude so varargs macros and system fprintf/vfprintf declarations are visible.
• Keep the manual formatted-stdio prototype block skipped for GCC+MinGW to avoid mingw-w64 static/extern conflicts.
• Update the manual stdlib/stdio prelude regression assertions for the new branch ordering.

Testing

• ./vnew fmt -w vlib/v/gen/c/cheaders.v vlib/v/gen/c/cheaders_manual_stdlib_decls_test.v
• ./v -g -keepc -o ./vnew cmd/v
• ./vnew -silent test vlib/v/gen/c/cheaders_manual_stdlib_decls_test.v
• ./vnew -skip-running -cc x86_64-w64-mingw32-gcc -os windows -o /tmp/v_va_arg_after.exe vlib/v/tests/va_arg_test.v
• ./vnew -silent vlib/v/gen/c/coutput_test.v
• ./vnew -silent vlib/v/compiler_errors_test.v
• ./vnew -cc x86_64-w64-mingw32-gcc -os windows -o /tmp/v_issue_27512_relaxed.exe examples/hello_world.v

Note: ./vnew -silent test vlib/v/ was attempted locally, but the default TCC-backed run on macOS arm64 failed unrelated tests before completion, including tcc_backtrace visibility in link_generated_c_files_test.v, TCC ARM asm support, and path-with-spaces codesign handling.

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