[gdb/testsuite] Use %progbits in gdb.arch/thumb*.S

In commit 0f2cd53cf4f ("[gdb/testsuite] Handle missing .note.GNU-stack") I
updated a gdb.arch/arm*.S test-case to use %progbits rather than @progbits,
but failed to do so for gdb.arch/thumb*.S.  Fix this oversight.

Tested on arm-linux-gnueabihf.