gcc: Set native_system_header_dir on aarch64-mingw

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.

gcc/config.gcc

diff --git a/gcc/config.gcc b/gcc/config.gcc
index 0947d63..b46cea8 100644
--- a/gcc/config.gcc
+++ b/gcc/config.gcc
@@ -1293,6 +1293,7 @@
 	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"