gdb: LoongArch: Silence warning about core file of lsx and lasx

In loongarch_iterate_over_regset_sections(), the second and third arguments
of the iterate_over_regset_sections_cb callback function should be the regset
size which is regsize * regnum. Otherwise when execute:

make check-gdb TESTS="gdb.base/corefile.exp"

there exists the following failed log:

  (gdb) core-file /home/fedora/community/gdb/build/gdb/testsuite/outputs/gdb.base/corefile/corefile.core
  [New LWP 531099]
  warning: Unexpected size of section `.reg-loongarch-lsx/531099' in core file.
  warning: Unexpected size of section `.reg-loongarch-lasx/531099' in core file.
  Core was generated by `/home/fedora/community/gdb/build/gdb/testsuite/outputs/gdb.base/corefile/corefile'.
  Program terminated with signal SIGABRT, Aborted.
  warning: Unexpected size of section `.reg-loongarch-lsx/531099' in core file.
  warning: Unexpected size of section `.reg-loongarch-lasx/531099' in core file.
  #0  0x00007ffff3081600 in __pthread_kill_implementation.constprop.0 () from /lib64/libc.so.6
  (gdb) FAIL: gdb.base/corefile.exp: core-file warning-free

Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>