explicitly use signed keyword to define intNN_t and derivative types

standing alone, both the signed and int keywords identify the same
type, a (signed) int. however the C language has an exception where,
when the lone keyword int is used to declare a bitfield, it's
implementation-defined whether the bitfield is signed or unsigned. C11
footnote 125 extends this implementation-definedness to typedefs, and
DR#315 extends it to other integer types (for which support with
bitfields is implementation-defined).

while reasonable ABIs (all the ones we support) define bitfields as
signed by default, GCC and compatible compilers offer an option
-funsigned-bitfields to change the default. while any signed types
defined without explicit use of the signed keyword are affected, the
stdint.h types, especially intNN_t, have a natural use in bitfields.
ensure that bitfields defined with these types always have the correct
signedness regardless of compiler & flags used.

see also GCC PR 83294.

include/alltypes.h.in

@@ -7,10 +7,10 @@ TYPEDEF _Addr regoff_t;
 TYPEDEF _Reg register_t;
 
 TYPEDEF signed char     int8_t;
-TYPEDEF short           int16_t;
-TYPEDEF int             int32_t;
-TYPEDEF _Int64          int64_t;
-TYPEDEF _Int64          intmax_t;
+TYPEDEF signed short    int16_t;
+TYPEDEF signed int      int32_t;
+TYPEDEF signed _Int64   int64_t;
+TYPEDEF signed _Int64   intmax_t;
 TYPEDEF unsigned char   uint8_t;
 TYPEDEF unsigned short  uint16_t;
 TYPEDEF unsigned int    uint32_t;