[gdb/testsuite] Speed up MACRO_AT_* calls

[gdb/testsuite] Speed up MACRO_AT_* calls

Currently, for each MACRO_AT_range or MACRO_AT_func in dwarf assembly the
following is done:
- $srcdir/$subdir/$srcfile is compiled to an executable using
  flags "debug"
- a new gdb instance is started
- the new executable is loaded.

This is inefficient, because the executable is identical within the same
Dwarf::assemble call.

Share the gdb instance in the same Dwarf::assemble invocation, which speeds
up a make check with RUNTESTFLAGS like this to catch all dwarf assembly
test-cases:
...
rtf=$(echo $(cd src/gdb/testsuite; find gdb.* -type f -name "*.exp" \
      | xargs grep -l Dwarf::assemble))
...
from:
...
real    1m39.916s
user    1m25.668s
sys     0m21.377s
...
to:
...
real    1m29.512s
user    1m17.316s
sys     0m19.100s
...

Tested on x86_64-linux.