Limit SH strncmp inline expansion (PR target/78460).

Limit SH strncmp inline expansion (PR target/78460).

GCC mainline built for sh4-linux-gnu runs out of memory building a
glibc test, which calls strncmp with very large constant size
argument, resulting in the SH inline strncmp expansion trying to
inline a fully unrolled expansion of strncmp for that size.

This patch limits that fully unrolled expansion to the case of less
than 32 bytes.  This is explicitly *not* trying to be optimal in any
way (very likely a lower threshold makes sense), just to limit enough
to avoid the out-of-memory issue in the glibc testsuite.

I have *not* run the GCC testsuite for SH.  I have verified that this
allows the glibc testsuite to build OK, with both GCC mainline and GCC
7 branch (and that the included test builds quickly with patched GCC,
runs out of memory with unpatched GCC).

	PR target/78460
	PR target/67712
gcc:
	* config/sh/sh-mem.cc (sh_expand_cmpnstr): Only unroll for
	constant count if that count is less than 32.

gcc/testsuite:
	* gcc.c-torture/compile/string-large-1.c: New test.

From-SVN: r251108