[gdb/testsuite] Add nopie in a few test-cases

When running test-case gdb.arch/i386-disp-step.exp with target board
unix/-m32/-fPIE/-pie we run into:
...
gdb compile failed, ld: i386-disp-step0.o: warning: relocation in read-only section `.text'
ld: warning: creating DT_TEXTREL in a PIE
...

Fix this by adding nopie in the compilation flags.

Likewise in a few other test-cases.

Tested on x86_64-linux.