testsuite: extend nopie handling to add -fno-pie to compiler flags

testsuite: extend nopie handling to add -fno-pie to compiler flags

Some older GCC, e.g. 7.5.0 on Ubuntu 18.04 need -fno-pie to be passed to
the compiler in addition to -no-pie to be passed to the linker for non-pie
code generation.

The gdb,nopie_flag is already documented as getting passed to the
compiler, not the linker.  Use that for the new -fno-pie compiler flag and
add a new gdb,nopie_ldflag for the existing -no-pie linker flag.

CAUTION: this might break existing board files that specify
         gdb,nopie_flag.  Affected board files need to rename
         gdb,nopie_flag into gdb,nopie_ldflag.