ldbl-128ibm-compat: Add syslog functions

Similarly to __vfprintf_internal and __vfscanf_internal, the internal
implementation of syslog functions (__vsyslog_internal) takes a
'mode_flags' parameter used to select the format of long double
parameters.  This patch adds variants of the syslog functions that set
'mode_flags' to PRINTF_LDBL_USES_FLOAT128, thus enabling the correct
printing of long double values on powerpc64le, when long double has IEEE
binary128 format (-mabi=ieeelongdouble).

Tested for powerpc64le.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Joseph Myers <joseph@codesourcery.com>
Reviewed-by: Paul E. Murphy <murphyp@linux.ibm.com>