Introduce a new built-in type: signed char.

Introduce a new built-in type: signed char.

C++ distinguish between "char", "signed char" and
"unsigned char", they are three independent types.

Fix QVariant behavior on ARM. On ARM "char" may mean
"unsigned char", but we depends on the sign during
a numerical conversions.

Change-Id: I610ce3fb88ed5964b67f3ae442d264fe16b2d261
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>