Enable -ffreestanding by default

[nfeske]

In the context of issue #5428, I encountered the following compile error when including the arm_neon.h header:

.../lib/gcc/aarch64-none-elf/12.3.0/include/stdint.h:9:26: error: no include path in which to search for stdint.h
    9 | # include_next <stdint.h>

The included stdint.h contains the following distinction:

#ifndef _GCC_WRAP_STDINT_H
#if __STDC_HOSTED__
# if defined __cplusplus && __cplusplus >= 201103L
#  undef __STDC_LIMIT_MACROS
#  define __STDC_LIMIT_MACROS
#  undef __STDC_CONSTANT_MACROS
#  define __STDC_CONSTANT_MACROS
# endif
# include_next <stdint.h>
#else
# include "stdint-gcc.h"
#endif
#define _GCC_WRAP_STDINT_H
#endif

__STDC_HOSTED__ is enabled by default. Hence, the code expects the presence of stdint.h supplied by a libc.
Only if the compiler was invoked with the argument -ffreestanding, this assumption does not apply.

Consequently, whenever including arm_neon.h for a native Genode component not using the libc, we would need to extend the component's target.mk with the following tweak for the corresponding compilation unit (e.g., main.cc):

CC_OPT_main += -ffreestanding

It goes without saying that this approach becomes impractical as soon as we embrace SIMD at more and more places.

Therefore, I propose to enable -ffreestanding by default, with the option to discharge the default for a given target.

In particular, all components using the libc won't use -ffreestanding.

[nfeske]

Changed by b455139