testsuite, gdb.btrace: fix 32-bit PIE false positives

For 32-bit position independent executables, GCC generates an extra call to

    __x86.get_pc_thunk.<reg>

which appears in the function call history.  It is correct to appear there
but this confuses the tests, which check for an expected sequence of
functions.

Build with nopie to avoid this complication.

gdb/testsuite/ChangeLog:
2020-12-04  Markus Metzger  <markus.t.metzger@intel.com>

	* gdb.btrace/exception.exp: Build with nopie.
	* gdb.btrace/function_call_history.exp: Likewise.
	* gdb.btrace/unknown_functions.exp: Likewise.