[gdb/testsuite] Add basic lmap for tcl < 8.6

[gdb/testsuite] Add basic lmap for tcl < 8.6

With test-case gdb.dwarf2/dw2-abs-hi-pc.exp and tcl 8.5, I run into:
...
ERROR: tcl error sourcing gdb/testsuite/gdb.dwarf2/dw2-abs-hi-pc.exp.
ERROR: invalid command name "lmap"
    while executing
"::gdb_tcl_unknown lmap i {dw2-abs-hi-pc.c dw2-abs-hi-pc-hello.c \
  dw2-abs-hi-pc-world.c} { expr { "$srcdir/$subdir/$i" } }"
...

Fix this by adding basic lmap support for tcl version < 8.6.

Tested on x86_64-linux.