This hopefully fixes an issue where the aarch64 builder sporadically
runs out of resources:
```
Testing lto-dso ... terminate called after throwing an instance of
'std::runtime_error'
what(): pthread_create has failed: Resource temporarily unavailable
collect2: fatal error: ld terminated with signal 6 [Aborted], core
dumped
compilation terminated.
```
mold 1.0.2 on i686 is broken with a failing assertion:
elf/../mold.h:315: std::pair<T*, bool>
mold::ConcurrentMap<T>::insert(std::string_view, mold::u64, const T&)
[with T = mold:🧝:SectionFragment<mold:🧝:I386>; std::string_view
= std::basic_string_view<char>; mold::u64 = long long unsigned int]:
Assertion `__builtin_popcount(nbuckets) == 1' failed.
Since 32-bit i686 is no longer supported by Fedora anyway, it doesn't
matter for us.