Remove duplicated code from __printf_fp_l, __printf_fphex, and __printf_size

Remove duplicated code from __printf_fp_l, __printf_fphex, and __printf_size

In __printf_fp_l, __printf_fphex, and __printf_size the blocks of code that are
used to read a double or long double argument, check for special values and
convert to multiprecision are similar.  When adding float128 support to libc,
more code would be duplicated to deal with the extra type.  This patch moves
the repetitive code to a macro which is now used by double and long double and
will be used for float128 when support is added, thus avoiding more
duplication.

Tested for powerpc64le and s390x.

	* stdio-common/printf_fp.c (PRINTF_FP_FETCH): New macro.
	(__printf_fp_l): Use the new macro to avoid duplicating code.
	* stdio-common/printf_fphex.c (PRINTF_FPHEX_FETCH): New macro.
	(__printf_fphex): Use the new macro to avoid duplicating code.
	* stdio-common/printf_size.c (PRINTF_SIZE_FETCH): New macro.
	(__printf_size): Use the new macro to avoid duplicating code.