Add -funsigned-char flag for opengl jni lib.

For ARM ABI, char was unsigned, but it's signed for X86 ABI.
Opengl JNI codes used type char as unsinged, it failed on
X86 platform.

Add -funsigned-char to fix this issue on x86 platform.
This flag also make maintaining compatibility for ARM platform.

Change-Id: I996a954e84190c36ff0f1a886d475ee0b8226831
Signed-off-by: Wang LiangX <liangx.wang@intel.com>