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.
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