x86_simd_caps: make mask value unsigned

fixes -fsanitize=integer warning:
runtime error: implicit conversion from type 'int' of value -1 (32-bit,
signed) to type 'unsigned int' changed the value to 4294967295 (32-bit,
unsigned)

Change-Id: I95d41aade78cea5e4f870a804d3f358c2cf618d7