Fix x86/mips breakage caused by kernel/uapi.

It seems that linux/stddef.h no longer defines NULL, and
crtbegin.o can not be compiled after the adoption of uapi.

This commit fix this issue by removing unnecessary isystem
and nostdinc option.  Now, the compilation command in
gen-platforms.sh should be similar to the build rules for
crtbegin.o in the bionic Android.mk.

Change-Id: Ic9c2588aac2d4f40054188f761cf95325a239094
diff --git a/build/tools/gen-platforms.sh b/build/tools/gen-platforms.sh
index efcb1f6..1cb839f 100755
--- a/build/tools/gen-platforms.sh
+++ b/build/tools/gen-platforms.sh
@@ -530,12 +530,10 @@
         log "Generating $ARCH C runtime object: $DST_FILE"
         (cd "$SRC_DIR" && $CC \
                  -I$SRCDIR/../../bionic/libc/include \
-                 -I$SRCDIR/../../bionic/libc/private \
                  -I$SRCDIR/../../bionic/libc/arch-common/bionic \
                  -I$SRCDIR/../../bionic/libc/arch-$ARCH/include \
-                 -isystem $SRCDIR/../../bionic/libc/kernel/uapi \
-                 -isystem $SRCDIR/../../bionic/libc/kernel/uapi/linux \
-                 -O2 -fpic -Wl,-r -nostdlib -nostdinc -o "$DST_DIR/$DST_FILE" $SRC_FILE) 1>>$TMPL 2>&1
+                 -DPLATFORM_SDK_VERSION=$API \
+                 -O2 -fpic -Wl,-r -nostdlib -o "$DST_DIR/$DST_FILE" $SRC_FILE) 1>>$TMPL 2>&1
         if [ $? != 0 ]; then
             dump "ERROR: Could not generate $DST_FILE from $SRC_DIR/$SRC_FILE"
             dump "Please see the content of $TMPL for details!"