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.

gcc/testsuite/ChangeLog

@@ -4,6 +4,7 @@
 	-march=v10 option on target ! march_option.
 	* gcc.target/cris/asm-v10.S, gcc.target/cris/inasm-v10.c,
 	gcc.target/cris/sync-1-v10.c: Similar.
+	* lib/target-supports.exp (effective_target_march_option): New.
 
 2020-01-19  Thomas König  <tkoenig@gcc.gnu.org>
 

gcc/testsuite/lib/target-supports.exp

@@ -2477,6 +2477,10 @@ proc check_effective_target_ti_c64xp { } {
     }]
 }
 
+# Check if a -march=... option is given, as part of (earlier) options.
+proc check_effective_target_march_option { } {
+    return [check-flags [list "" { *-*-* } { "-march=*" } { "" } ]]
+}
 
 proc check_alpha_max_hw_available { } {
     return [check_runtime alpha_max_hw_available {