Provide a sensible default value for native_system_header_dir, namely
/mingw/include, on aarch64-mingw. This is in line with the expectations
for mingw file locations, and is already set on both x86- and
x86_64-mingw.
gcc/ChangeLog:
* config.gcc (aarch64-*-mingw*): Set native_system_header_dir.
tm_file="${tm_file} mingw/winnt.h"
tm_file="${tm_file} mingw/winnt-dll.h"
tmake_file="${tmake_file} aarch64/t-aarch64"
+ native_system_header_dir=/mingw/include
target_gtfiles="$target_gtfiles \$(srcdir)/config/mingw/winnt.cc"
target_gtfiles="$target_gtfiles \$(srcdir)/config/mingw/winnt-dll.cc"
extra_options="${extra_options} mingw/cygming.opt mingw/mingw.opt"