backends: allocate enough stace for null terminator

backends: allocate enough stace for null terminator

`gcc-15` added a new warning in https://gcc.gnu.org/PR115185:

    i386_regs.c:88:11: error: initializer-string for array of 'char' is too long [-Werror=unterminated-string-initialization]
       88 |           "ax", "cx", "dx", "bx", "sp", "bp", "si", "di", "ip"
          |           ^~~~

`elfutils` does not need to store '\0'. We could either initialize the
arrays with individual bytes or allocate extra byte for null.

This change initializes the array bytewise.

	* backends/i386_regs.c (i386_register_info): Initialize the
	array bytewise to fix gcc-15 warning.
	* backends/x86_64_regs.c (x86_64_register_info): Ditto.

Signed-off-by: Sergei Trofimovich <slyich@gmail.com>