[gdb/testsuite] Don't use string cat in gdb.dwarf2/dw2-abs-hi-pc.exp

Test-case gdb.dwarf2/dw2-abs-hi-pc.exp uses string cat:
...
set sources [lmap i $sources { string cat "${srcdir}/${subdir}/" $i }]
...
but that's only supported starting tcl 8.6.

Fix this by using "expr" instead:
...
set sources [lmap i $sources { expr { "$srcdir/$subdir/$i" } }]
...

Tested on x86_64-linux.