testsuite: Fix cc-with-tweaks.sh being executed in the wrong shell

The cc-with-tweaks.sh script needs to be executed with bash.  When
trying to run this:

  make check RUNTESTFLAGS="--target_board=dwarf4-gdb-index" TESTS="gdb.base/return.exp"

I get:

  gdb compile failed, /home/emaisin/src/binutils-gdb/gdb/contrib/cc-with-tweaks.sh: 174: /home/emaisin/src/binutils-gdb/gdb/contrib/cc-with-tweaks.sh: Bad substitution

The reason is that the board files execute cc-with-tweaks.sh using
/bin/sh, which points to dash on my machine.  Remove the /bin/sh part
and let the shebang choose the right interpreter.

gdb/testsuite/ChangeLog:

	* boards/cc-with-tweaks.exp: Don't call cc-with-tweaks.sh
	through /bin/sh.
	* boards/dwarf4-gdb-index.exp: Likewise.
	* boards/fission-dwp.exp: Likewise.