Fix unsigned comparison warning

Fix unsigned comparison warning

When -mfloat-abi=hard support was added, a cast went missing that
used to silence a warning in common code:

/usr/lib/gcc-snapshot/bin/g++  -fno-PIE -c   -g -O2   -DIN_GCC  -DCROSS_DIRECTORY_STRUCTURE   -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -I. -I. -I../../gcc/gcc -I../../gcc/gcc/. -I../../gcc/gcc/../include -I../../gcc/gcc/../libcpp/include -I../../gcc/gcc/../libcody  -I../../gcc/gcc/../libdecnumber -I../../gcc/gcc/../libdecnumber/dpd -I../libdecnumber -I../../gcc/gcc/../libbacktrace   -o builtins.o -MT builtins.o -MMD -MP -MF ./.deps/builtins.TPo ../../gcc/gcc/builtins.cc
In file included from ./tm.h:21,
                 from ../../gcc/gcc/backend.h:28,
                 from ../../gcc/gcc/builtins.cc:27:
../../gcc/gcc/builtins.cc: In function 'int apply_args_size()':
../../gcc/gcc/config/csky/csky.h:421:13: error: comparison of unsigned expression in '>= 0' is always true [-Werror=type-limits]
  421 |   (((REGNO) >= CSKY_FIRST_PARM_REGNUM                        \
../../gcc/gcc/builtins.cc:1444:13: note: in expansion of macro 'FUNCTION_ARG_REGNO_P'
 1444 |         if (FUNCTION_ARG_REGNO_P (regno))
      |             ^~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
make[1]: *** [Makefile:1146: builtins.o] Error 1

The needed (int) cast is even mentioned in the comment above, so reinstate
it here.

gcc/ChangeLog:
	* config/csky/csky.h (FUNCTION_ARG_REGNO_P): Cast REGNO to (int)
	to prevent warning.