gdb: Minor cleanup in some gdb.arch/* tests

gdb: Minor cleanup in some gdb.arch/* tests

A small number of tests incorrectly tried to pass -Wa,-g through to
GCC as an extra compile time flag, either to gdb_compile or
prepare_for_testing.

The problem is that the syntax used for passing the flags was
incorrect, and as a result these extra flags were being ignored.
Luckily, the 'debug' flag was being passed in each case anyway, which
means that the '-g' flag would already be added.

Given that all these tests pass 'debug', and the invalid flag has been
ignored for some time, I'm just removing the flags in this commit.

I've also changed the tests from using gdb_compile to
prepare_for_testing, which allows some extra code to be removed from a
couple of tests scripts.

There should be no change in the test results after this commit.

gdb/testsuite/ChangeLog:

	* gdb.arch/amd64-disp-step-avx.exp: Remove unneeded assembler flag
	option, syntax was wrong anyway.
	* gdb.arch/arm-disp-step.exp: Likewise.
	* gdb.arch/sparc64-regs.exp: Likewise.
	* gdb.arch/amd64-disp-step.exp: Remove unneeded assembler flag
	option, syntax was wrong anyway, switch to use
	prepare_for_testing.
	* gdb.arch/i386-disp-step.exp: Likewise.