arm: Allow -mcpu and -march options to be unset

The compiler will warn if the architectural specification derived from
a -mcpu option is not the same as that specified by -march.  This is
because it was never intended that the two should be used at the same
time: -mcpu=<name> is supposed to be shorthand for -mtune=<name>
-march=arch-of(<name>).

Unfortunately, there are times when the two options passed to the
compiler may come from distinct sources: one example is makefiles
which accumulate options; another is the testsuite itself, when some
tests require a particular architecture setting to be useful - only
running the tests when the compiler/testsuite configuration exactly
matched the requirements would make regression testing especially hard
(we have too many permutations).

So this patch allows a user to cancel any earlier setting of a
particular flag and to make the compiler behave as though it was never
passed.  The intended usecase is (sources of options are shown in
parenthesis, but that's just for grouping:

 (-march=armv7-a+simd) (-march=unset -mcpu=cortex-m33)

The option processing logic will now simplify this to:

 -mcpu=cortex-m33

A useful corollary of this is that

 -march=armv7-a -march=unset

will now cause the compiler to behave as though neither the
architecture nor the CPU was ever set and to default back to the
configure-time settings.

gcc/ChangeLog:

	* config/arm/arm.h (OPTION_DEFAULT_SPECS): Allow -mcpu and -march
	to be unset.
	(ARCH_CPU_CLEANUP_SPECS): Likewise
	(DRIVER_SELF_SPECS): Add ARCH_CPU_CLEANUP_SPECS
	* doc/invoke.texi (arm: -mcpu= and -march=): Document use of 'unset'.

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