#ifdefs inside variable args macro breaks the parser

[vesim987]

Minimal repro:

#define va_args_expand(...) __VA_ARGS__

va_args_expand (
#ifdef __riscv
    #warning riscv
#else
    #warning other
#endif
)

causes multiple errors:

foo.h:3:1: error: expected external declaration
va_args_expand (
^
foo.h:1:29: note: expanded from here
#define va_args_expand(...) __VA_ARGS__
                            ^
foo.h:5:5: note: expanded from here
    #warning riscv
    ^
foo.h:3:1: error: unknown type name 'warning'
va_args_expand (
^
foo.h:1:29: note: expanded from here
#define va_args_expand(...) __VA_ARGS__
                            ^
foo.h:5:6: note: expanded from here
    #warning riscv
     ^
foo.h:5:19: error: expected ';' before end of file
    #warning riscv
                  ^

This is reduction of https://github.com/raspberrypi/pico-sdk/blob/master/src/rp2350/pico_platform/include/pico/platform.h#L93-L110