PowerPC: Fix nearbyint/nearbyintf result for FE_DOWNWARD

This patch fixes the powerpc32 optimized nearbyint/nearbyintf bogus
results for FE_DOWNWARD rounding mode.  This is due wrong instructions
sequence used in the rounding calculation (two subtractions instead of
adition and a subtraction).

Fixes BZ#16815.
PowerPC: define _CALL_ELF if compiler does not

This patch makes the configure adds -D_CALL_ELF=1 when compiler does
not define _CALL_ELF (versions before powerpc64le support).  It cleans
up compiler warnings on old compiler where _CALL_ELF is not defined
on powerpc64(be) builds.

It does by add a new config.make variable for configure-deduced
CPPFLAGS and accumulate into that (confix-extra-cppflags).  It also
generalizes libc_extra_cflags so it accumulates in sysdeps configure
fragmenets.