linux-user: move target_flat.h to target subdirs

linux-user: move target_flat.h to target subdirs

This makes target_flat.h behave like every other target_xxx.h header.
It also makes it actually work -- while the current header says adding
a header to the target subdir overrides the common one, it doesn't.
This is for two reasons:
* meson.build adds -Ilinux-user before -Ilinux-user/$arch
* the compiler search path for "target_flat.h" looks in the same dir
  as the source file before searching -I paths.

This can be seen with the xtensa port -- the subdir settings aren't
used which breaks stack setup.

Move it to the generic/ subdir and add include stubs like every
other target_xxx.h header is handled.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230129004625.11228-1-vapier@gentoo.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>