__patchable_function_entries: don't use linkage field 'unique' with -no-integrated-as

.section name, "flags"G, @type, GroupName[, linkage]

As of binutils 2.33, linkage cannot be 'unique'.  For integrated
assembler, we use both 'o' flag and 'unique' linkage to support
--gc-sections and COMDAT with lld.

https://sourceware.org/ml/binutils/2019-11/msg00266.html