LLD 13 and GNU ld 2.37 support -z start-stop-gc which allows garbage
collection of C identifier name sections despite the __start_/__stop_
references. GNU ld before 2015-10 had the behavior as well. Simply set
the retain attribute so that GCC 11 (if configure-time binutils is 2.36
or newer)/Clang 13 will set the SHF_GNU_RETAIN section attribute to
prevent garbage collection.
Without the patch, there are linker errors with -z start-stop-gc
(LLD default) when -Wl,--gc-sections is used:
```
ld.lld: error: undefined symbol: __start___eap
>>> referenced by eap.c
>>> src/eap.o:(eap_init)
```
The remain attribute will not be needed if the metadata sections are
referenced by code directly.
Apparently the intention was for the dependent module's name to appear
in the variable name resulting from using IWD_MODULE_DEPENDS, so the
dependencies all have unique names (apparently not critical).
