[gdb/testsuite] Fix incorrect string concat in jit-elf.exp

[gdb/testsuite] Fix incorrect string concat in jit-elf.exp

When running test-case gdb.base/jit-elf.exp with target board
cc-with-gdb-index, we get:
...
spawn -ignore SIGHUP gdb/contrib/cc-with-tweaks.sh -i gcc \
  -fno-stack-protector src/gdb/testsuite/gdb.base/jit-elf-main.c \
  -fdiagnostics-color=never -DATTACH=1 -DLOAD_ADDRESS=0x7000000 \
  -DLOAD_INCREMENT=0x1000000 -g -lm \
  -o outputs/gdb.base/jit-elf/jit-elf-main"-attach"^M
outputs/gdb.base/jit-elf/.tmp/jit-elf-main-attach: \
  No such file or directory.^M
output is:
outputs/gdb.base/jit-elf/.tmp/jit-elf-main-attach: \
  No such file or directory.^M

gdb compile failed, outputs/gdb.base/jit-elf/.tmp/jit-elf-main-attach: \
  No such file or directory.
UNTESTED: gdb.base/jit-elf.exp: failed to compile jit-elf-main"-attach"
...

The problem is a string concat in jit-elf.exp:
...
  ${main_binfile}"-attach"
...
which is intended to generate string 'jit-elf-main-attach' but instead
generates string 'jit-elf-main"-attach"'.

Fix this by using "${main_binfile}-attach" instead.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-12  Tom de Vries  <tdevries@suse.de>

	* gdb.base/jit-elf.exp: Fix string concat.