gas:LoongArch: Fix segment error in compilation due to too long symbol name.

  Change "char buffer[8192];" into "char *buffer =
  (char *) malloc(1000 +  6 * len_str);" in function
  loongarch_expand_macro_with_format_map.

  gas/
    * config/tc-loongarch.c

  include/
    * opcode/loongarch.h

  opcodes/
    * loongarch-coder.c