testsuite: effective_target_march_option: support checking for -march=*

	* lib/target-supports.exp (effective_target_march_option): New.

I see no (other) way to, depending on the absence of an option,
add an option for a specific target.

For gcc.dg/torture/pr26515.c and cris-elf, you get an error for
supplying multiple (different) -march=... options (where that
error is desirable), like testing cris-elf with
RUNTESTFLAGS=--target_board=cris-sim/arch=v8, where otherwise
-march=v10 and -march=v8 will both be given, and the test would
fail.

For historians, this was accidentally misordered and committed after
the (first) patch using march_option.  Oops.

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