s390: s390_machinemode leak

This resolves the following memory leak reported by ASAN:

Direct leak of 17 byte(s) in 1 object(s) allocated from:
    #0 0x3ffb32fbb1d in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    bminor#1 0x2aa149861cf in xmalloc ../../libiberty/xmalloc.c:149
    bminor#2 0x2aa149868ff in xstrdup ../../libiberty/xstrdup.c:34
    bminor#3 0x2aa1312391f in s390_machinemode ../../gas/config/tc-s390.c:2241
    bminor#4 0x2aa130ddc7b in read_a_source_file ../../gas/read.c:1293
    bminor#5 0x2aa1304f7bf in perform_an_assembly_pass ../../gas/as.c:1223
    bminor#6 0x2aa1304f7bf in main ../../gas/as.c:1436
    bminor#7 0x3ffb282be35 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    bminor#8 0x3ffb282bf33 in __libc_start_main_impl ../csu/libc-start.c:360
    bminor#9 0x2aa1305758f  (/home/jremus/git/binutils/build-asan/gas/as-new+0x2d5758f) (BuildId: ...)

gas/
	* config/tc-s390.c (s390_machinemode): Free mode_string before
	returning.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>

Author: Jens Remus

gas/config/tc-s390.c

@@ -2287,6 +2287,7 @@ s390_machinemode (int ignore ATTRIBUTE_UNUSED)
 	s390_setup_opcodes ();
     }
 
+  xfree (mode_string);
   demand_empty_rest_of_line ();
 }