configure: Use a different aarch64 feature in Clang workaround

We use a dummy aarch64 feature to work around an issue in older
Clang, where an .arch line such as ".arch armv8.2-a" doesn't take
effect immediately, while one like ".arch armv8.2-a+feature" works.

Previously, we used "crc" for this dummy feature to add (as an
old feature that would be supported widely by old toolchains).
But as we may want to actually use crc features and detect whether
they are supported, we may want to switch to another feature.

Use the "fp" feature instead, for the purposes of this extra
feature in the .arch lines. (The "fp" feature indicates floating
point support, which is implicitly part of the baseline feature
set anyway.)

configure

@@ -6451,10 +6451,10 @@ if enabled aarch64; then
         # also had a bug (https://github.com/llvm/llvm-project/issues/32220)
         # causing a plain ".arch <level>" to not have any effect unless it
         # had an extra "+<feature>" included - but it was activated on the next
-        # ".arch_extension" directive. Check if we can include "+crc" as dummy
+        # ".arch_extension" directive. Check if we can include "+fp" as dummy
         # feature to make the .arch directive behave as expected and take
         # effect right away.
-        check_arch_level "${as_arch_level}+crc"
+        check_arch_level "${as_arch_level}+fp"
     fi
 
     enabled armv8 && check_insn armv8 'prfm   pldl1strm, [x0]'