LoongArch: Fix ld "undefined reference" error with --enable-shared

  Because _bfd_read_unsigned_leb128 is hidden visibility, so it can't
  be referenced out of shared object.

  The new function loongarch_get_uleb128_length just used to call
  _bfd_read_unsigned_leb128.

bfd/ChangeLog:

	* elfxx-loongarch.c (loongarch_get_uleb128_length): New function.
	* elfxx-loongarch.h (loongarch_get_uleb128_length): New function.

gas/ChangeLog:

	* config/tc-loongarch.c (md_apply_fix): Use
	loongarch_get_uleb128_length.