gas: fix a testcase broken by new ZSTD support

gas: fix a testcase broken by new ZSTD support

The commit 1369522f36eece1b37139a81f7f2139ba3915172 ("Recognize the new ELF
compression type for ZSTD.") added the new ELF compression type but it
accidentally broke a GAS testcase.  Since testing for the section type
"2048" (SHF_COMPRESSED) is not going to be portable in the long term, it
now tests SHF_LINK_ORDER ("128") instead.

Using SHF_LINK_ORDER (with possibly sh_link == 0) is an idea by Jan Beulich.

gas/ChangeLog:

	* testsuite/gas/elf/section10.s: Use SHF_LINK_ORDER to test
	mixed numeric and alpha values.
	* testsuite/gas/elf/section10.d: Reflect the change above.