[bitops] Provide an explicit operand size for bit test instructions

[bitops] Provide an explicit operand size for bit test instructions

Recent versions of the GNU assembler (observed with GNU as 2.35 on
Fedora 33) will produce a warning message

  Warning: no instruction mnemonic suffix given and no register
  operands; using default for `bts'

The operand size affects only the potential range for the bit number.
Since we pass the bit number as an unsigned int, it is already
constrained to 32 bits for both i386 and x86_64.

Silence the assembler warning by specifying an explicit 32-bit operand
size (and thereby matching the choice that the assembler would
otherwise make automatically).

Signed-off-by: Michael Brown <mcb30@ipxe.org>