[gdb/testsuite] Clean up stale exec in gdb_compile_pascal

[gdb/testsuite] Clean up stale exec in gdb_compile_pascal

When running a pascal test with the stabs target board:
...
$ test=gdb.pascal/case-insensitive-symbols.exp
$ cd build/gdb/testsuite
$ make check RUNTESTFLAGS="$test --target_board=stabs"
...
we get:
...
nr of untested testcases         1
nr of unsupported tests          1
...
due to:
...
Error: Illegal parameter: -gstabs+^M
Error: /usr/bin/ppcx64 returned an error exitcode^M
...

OTOH, when running the same pascal test without the stabs target board:
...
$ make check RUNTESTFLAGS="$test"
...
we get:
...
nr of expected passes            20
...

But when subsequently again running with the stabs target board:
...
$ make check RUNTESTFLAGS="$test --target_board=stabs"
...
we now get:
...
nr of expected passes            20
...

The problem is that gdb_compile_pascal determines success based on existence
of the exec after compilation:
...
    if ![file exists $destfile] {
        unsupported "Pascal compilation failed: $result"
        return "Pascal compilation failed."
    }
...
without removing the exec before compilation, which allows a stale exec to
make it seem as if compilation has succeeded.

Fix this by removing the stale exec before compilation.

gdb/testsuite/ChangeLog:

2019-08-20  Tom de Vries  <tdevries@suse.de>

	* lib/pascal.exp (gdb_compile_pascal): Remove $destfile before
	compilation.