gdb: LoongArch: Fix code style issues

Fix some code style issues suggested by Tom Tromey and Andrew Burgess,
thank you.

(1) Put an introductory comment to explain the purpose for some functions.

(2) Modify the the attribute code to make it portable.

(3) Remove globals and pass pointers to locals.

(4) Remove "*" in the subsequent comment lines.

(5) Put two spaces before "{" and "}".

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