[gdb/testsuite] Use pie instead of -fpie/-pie

I noticed two test-cases where -fpie is used.  Using the canonical pie option
will usually get one -fPIE instead.

That choice is justified here in gdb_compile:
...
  # For safety, use fPIE rather than fpie. On AArch64, m68k, PowerPC
  # and SPARC, fpie can cause compile errors due to the GOT exceeding
  # a maximum size.  On other architectures the two flags are
  # identical (see the GCC manual). Note Debian9 and Ubuntu16.10
  # onwards default GCC to using fPIE.  If you do require fpie, then
  # it can be set using the pie_flag.
  set flag "additional_flags=-fPIE"
...

There is no indication that using -fpie rather than -fPIE is on purpose, so
use pie instead.

Tested on x86_64-linux.