testsuite: [arm] add -mthumb to arm/cortex-m55 tests

[Christophe Lyon]

The recently added arm/cortex-m55-* tests fail if the compiler does
not default to thumb code.

This patch adds -mthumb to the dg-additional-options, which fixes the
problem in most cases.

When running the tests with an incompatible -march= value, the tests
also fail because it conflicts with -mcpu=cortex-m55. I tried to add
-march=armv8.1-m.main+XXX but found it awkward to get the right value
for XXX to match what each test is adding to cortex-m55, so I propose
to consider adding -mthumb as good enough.

Maybe we can/should add dg-skip-if for -march?

OK with a suitable changelog?

Thanks,

Christophe
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