Allow abort PLT references in libc.so for SH.

Given my patch
<https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00965.html> for the
ICEs building a glibc string function test for SH, the testsuite can
build completely for SH with GCC 7 and mainline.  However, there is a
test failure that does not appear for GCC 6: check-localplt fails
because of an abort PLT reference in libc.so.

Given the lack of a trap insn pattern for SH, it seems unavoidable
that the compiler might sometimes generate abort calls, and such abort
calls (generated from __builtin_trap when there is no trap insn
pattern) will be unaffected by the normal mapping to __GI_abort for
calls within glibc.  Thus, this patch allows (but does not require) an
abort PLT reference in libc.so for SH.

Tested for sh4-linux-gnu with build-many-glibcs.py (GCC 7, with my
patch applied).

	* sysdeps/unix/sysv/linux/sh/localplt.data: Allow abort in
	libc.so.