gdb/csky fix build error in ubuntu20_04

gdb/csky fix build error in ubuntu20_04

build error in: https://builder.sourceware.org/buildbot/#/builders/170/builds/246
...
../../binutils-gdb/gdb/csky-linux-tdep.c: In function ‘void
csky_supply_fregset(const regset*, regcache*, int, const void*, size_t)’:
../../binutils-gdb/gdb/csky-linux-tdep.c:194:18: error: format ‘%ld’
expects argument of type ‘long int’, but argument 2 has type ‘size_t’
{aka ‘unsigned int’} [-Werror=format=]
   194 |       warning (_("Unknow size %ld of section .reg2, can not get
value"
       |
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   195 |    " of float registers."), len);
...

Fix it via using %s vs pulongest suggested by Tom.