[gdb/testsuite] Fix dwo path in fission-*.S

[gdb/testsuite] Fix dwo path in fission-*.S

[ Using $build for /home/vries/gdb_versions/devel/build to make things a bit
more readable. ]

When using make check// to run test-case gdb.dwarf2/fission-base.exp:
...
( cd $build/gdb; make check//unix RUNTESTFLAGS="fission-base.exp" )
...
we run into:
...
(gdb) file \
  $build/gdb/testsuite.unix/outputs/gdb.dwarf2/fission-base/fission-base^M
Reading symbols from \
  $build/gdb/testsuite.unix/outputs/gdb.dwarf2/fission-base/fission-base...^M
warning: Could not find DWO CU \
  $build/gdb/testsuite.1/outputs/gdb.dwarf2/fission-base/fission-base.dwo \
  (0x807060504030201) referenced by CU at offset 0xc7 [in module \
  $build/gdb/testsuite.unix/outputs/gdb.dwarf2/fission-base/fission-base]^M
...

The problem is that the executable refers to the dwo file using path name
$build/gdb/testsuite.1/outputs/gdb.dwarf2/fission-base/fission-base.dwo,
while the actual dwo file is at
$build/gdb/testsuite.unix/outputs/gdb.dwarf2/fission-base/fission-base.dwo.

This is caused by this trick in fission-base.S:
...
 #define XSTR(s) STR(s)
 #define STR(s) #s
   ...
   .asciz XSTR(DWO)        # DW_AT_GNU_dwo_name
...
and:
...
$ echo | gcc -E -dD - | grep "define unix"
...

I used this trick to avoid doing additional_flags=-DDWO=\"$dwo\", since I was
concerned that there could be quoting issues.

However, I've found other uses of this pattern, f.i. in
gdb/testsuite/gdb.base/corefile-buildid.exp:
...
  additional_flags=-DSHLIB_NAME=\"$dlopen_lib\"]
...

So, fix this by:
- using additional_flags=-DDWO=\"$dwo\" and
- using plain DWO instead of XSTR(DWO)

Likewise in other gdb.dwarf2/fission*.exp test-cases.

Tested on x86_64-linux, using make check//unix.

gdb/testsuite/ChangeLog:

2021-09-01  Tom de Vries  <tdevries@suse.de>

	PR testsuite/28298
	* gdb.dwarf2/fission-base.S: Use DWO instead of XSTR(DWO).
	* gdb.dwarf2/fission-loclists-pie.S: Same.
	* gdb.dwarf2/fission-loclists.S: Same.
	* gdb.dwarf2/fission-reread.S: Same.
	* gdb.dwarf2/fission-base.exp: Use additional_flags=-DDWO=\"$dwo\".
	* gdb.dwarf2/fission-loclists-pie.exp: Same.
	* gdb.dwarf2/fission-loclists.exp: Same.
	* gdb.dwarf2/fission-reread.exp: Same.