[llvm] [RISCV] Add a new subtarget feature for throttled FP64 vector performance (PR #162399)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-quick` running on `linaro-clang-aarch64-quick` while building `llvm` at step 5 "ninja check 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/65/builds/23808

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 5 (ninja check 1) failure: stage 1 checked (failure)
******************** TEST 'Clangd Unit Tests :: ./ClangdTests/323/333' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests-Clangd Unit Tests-2510779-323-333.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=333 GTEST_SHARD_INDEX=323 /home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests
--

Script:
--
/home/tcwg-buildbot/worker/clang-aarch64-quick/stage1/tools/clang/tools/extra/clangd/unittests/./ClangdTests --gtest_filter=StdLibTests.StdLibDocComments
--
ASTWorker building file /clangd-test/foo.cc version null with command 
[/clangd-test]
clang -ffreestanding -isystem/clangd-test/stdlib /clangd-test/foo.cc
Driver produced command: cc1 -cc1 -triple aarch64-unknown-linux-gnu -fsyntax-only -disable-free -clear-ast-before-backend -main-file-name foo.cc -mrelocation-model pic -pic-level 2 -pic-is-pie -mframe-pointer=non-leaf -fmath-errno -ffp-contract=on -fno-rounding-math -mconstructor-aliases -ffreestanding -enable-tlsdesc -target-cpu generic -target-feature +v8a -target-feature +fp-armv8 -target-feature +neon -target-abi aapcs -debugger-tuning=gdb -fdebug-compilation-dir=/clangd-test -fcoverage-compilation-dir=/clangd-test -resource-dir lib/clang/22 -isystem /clangd-test/stdlib -internal-isystem lib/clang/22/include -internal-isystem /usr/local/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -ferror-limit 19 -fno-signed-char -fgnuc-version=4.2.1 -fskip-odr-check-in-gmf -fcxx-exceptions -fexceptions -no-round-trip-args -target-feature -fmv -faddrsig -D__GCC_HAVE_DWARF2_CFI_ASM=1 -x c++ /clangd-test/foo.cc
Building first preamble for /clangd-test/foo.cc version null
../llvm/clang-tools-extra/clangd/unittests/StdLibTests.cpp:188: Failure
Value of: Server.blockUntilIdleForTest()
  Actual: false
Expected: true

Built preamble of size 737048 for file /clangd-test/foo.cc version null in 16.14 seconds
Indexing c++17 standard library in the context of /clangd-test/foo.cc
indexed preamble AST for /clangd-test/foo.cc version null:
  symbol slab: 0 symbols, 120 bytes
  ref slab: 0 symbols, 0 refs, 128 bytes
  relations slab: 0 relations, 24 bytes
indexed preamble AST for /clangd-test/foo.cc version :
  symbol slab: 3 symbols, 4912 bytes
  ref slab: 0 symbols, 0 refs, 128 bytes
  relations slab: 0 relations, 24 bytes
Indexed c++17 standard library: 3 symbols, 0 filtered
Build dynamic index for header symbols with estimated memory usage of 8820 bytes

../llvm/clang-tools-extra/clangd/unittests/StdLibTests.cpp:188
Value of: Server.blockUntilIdleForTest()
  Actual: false
Expected: true



********************


```

</details>

https://github.com/llvm/llvm-project/pull/162399