[gdb/testsuite] Fix nopie test-cases with target board unix/-fPIE/-pie

[gdb/testsuite] Fix nopie test-cases with target board unix/-fPIE/-pie

Compilers default to either PIE or no-PIE executables.

In order to test PIE executables with a compiler that produces non-PIE by
default, we can use target board unix/-fPIE/-pie, which set the multilib_flags
of the target board to "-fPIE -pie".

Likewise, we can use target board unix/-fno-PIE/-no-pie with a compiler that
produces PIE by default.

The target board unix/-fno-PIE/-no-pie has a potential problem when compiling
shared libs, because the multilib_flags will override the attempts of
gdb_compile_shlib to compile with -fPIC.  This is taken care of by running the
body of gdb_compile_shlib wrapped in with_PIE_multilib_flags_filtered.

The target board unix/-fPIE/-pie has a problem with nopie compilations.  The
current approach is to do the compilation hoping for the best, and if we find
out that the resulting executable is PIE despite specifying nopie, we error
out with the standard error message "nopie failed to prevent PIE executable".

That however does not work for hard-coded assembly nopie test-cases, which will
just noisily refuse to compile:
...
ld: amd64-disp-step0.o: relocation R_X86_64_32S against `.text' can not be \
  used when making a PIE object; recompile with -fPIE^M
...

Fix this in gdb_compile by filtering out the PIE settings in the target board
multilib_flags when pie or nopie is specified.

Tested on x86_64-linux.