gdb: LoongArch: fix failed testcases in gdb.base/align-c.exp

When execute the following command on LoongArch:

  make check-gdb TESTS="gdb.base/align-c.exp"

there exist some failed testcases:

  ...
  FAIL: gdb.base/align-c.exp: print _Alignof(struct align_pair_long_double_x_float)
  FAIL: gdb.base/align-c.exp: print _Alignof(struct align_pair_long_double_x_double)
  FAIL: gdb.base/align-c.exp: print _Alignof(struct align_pair_long_double_x_long_double)
  ...

According to LoongArch ELF ABI specification [1], set the target data types
of floating-point to fix the above failed testcases.

[1] https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html

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