gas: Update fr_literal access in frag for GCC 10

gas: Update fr_literal access in frag for GCC 10

When access fr_literal in

struct frag {
  ...
  /* Data begins here.  */
  char fr_literal[1];
};

with

char *buf = fragp->fr_fix + fragp->fr_literal;

GCC 10 gave

gas/config/tc-csky.c: In function ‘md_convert_frag’:
gas/config/tc-csky.c:4507:9: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
 4507 |  buf[1] = BYTE_1 (CSKYV1_INST_SUBI | (7 << 4));
      |         ^

Change

  char *buf = fragp->fr_fix + fragp->fr_literal;

to

  char *buf = fragp->fr_fix + &fragp->fr_literal[0];

to silence GCC 10 warning.

	* config/tc-csky.c (md_convert_frag): Replace fragp->fr_literal
	with &fragp->fr_literal[0].
	* config/tc-microblaze.c (md_apply_fix): Likewise.
	* config/tc-sh.c (md_convert_frag): Likewise.