gdb: LoongArch: Define LOONGARCH_LINUX_NUM_GREGSET as 45

gdb: LoongArch: Define LOONGARCH_LINUX_NUM_GREGSET as 45

LOONGARCH_LINUX_NUM_GREGSET should be defined as 45 (32 + 1 + 1 + 11)
due to reserved 11 for extension in glibc, otherwise when execute:

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

there exists the following failed testcase:

  (gdb) core-file /home/loongson/build.git/gdb/testsuite/outputs/gdb.base/corefile/corefile.core
  [New LWP 7742]
  warning: Unexpected size of section `.reg/7742' in core file.
  Core was generated by `/home/loongson/build.git/gdb/testsuite/outputs/gdb.base/corefile/corefile'.
  Program terminated with signal SIGABRT, Aborted.
  warning: Unexpected size of section `.reg/7742' in core file.
  #0  0x000000fff76f4e24 in raise () from /lib/loongarch64-linux-gnu/libc.so.6
  (gdb) FAIL: gdb.base/corefile.exp: core-file warning-free

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