lang/rust: fix build on armv7

We have previously (cf. PR 282663) disabled the has_thread_local feature
on armv7 to work around an rtld bug. The bug was fixed with D42415, but
the workaround remained. Earlier this year, rust started to fail to
build due to the workaround, as the fallback TLS implementation is
limited to the number of pthread TLS keys (256) and rust has started to
consume more and more of them. While a temporary workaround reducing
the hunger for TLS keys was implemented in 2026Q2, we can actually just
reenable has_thread_local to go back to how things are supposed to work.

PR: 294545
Approved by: mikael (rust)
MFH: no