[RS6000] PR79144, cmpstrnsi optimization breaks glibc

[RS6000] PR79144, cmpstrnsi optimization breaks glibc

glibc compiled with current gcc-7 fails one test due to strcmp and
strncmp appearing in the PLT.  This is because the inline expansion of
those functions falls back to a function call, but doesn't use the asm
name for the call.

	PR target/79144
	* config/rs6000/rs6000.c (expand_strn_compare): Get the asm name
	for strcmp and strncmp from corresponding builtin decl.

From-SVN: r244659