gas: avoid GCC 10 warning stringop-overflow in tc-bpf.c

gas: avoid GCC 10 warning stringop-overflow in tc-bpf.c

The GAS struct frag ends with a field `fr_literal' whose purpose is to
mark the begining of the frag's data:

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

The code in gas/config/tc-bpf.c recently committed:

where = fixP->fx_frag->fr_literal + fixP->fx_where;
where[1] = target_big_endian ? 0x01 : 0x10;

triggers the stringop-overflow warning in GCC 10+, since the compiler
assumes the size of the modified buffer is 1 byte.  This patch
slightly modifies the code to make tc-bpf.c buildable with GCC 10+.

2020-06-05  Jose E. Marchesi  <jose.marchesi@oracle.com>

	* config/tc-bpf.c (md_apply_fix): Avoid GCC 10 warning
	stringop-overflow.