[llvm] flang-rt: improve endian conversion performance (PR #218302)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 332 tests passed
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### flang-rt
<details>
<summary>flang-rt.Driver/endian.f90</summary>

```
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 8
/home/gha/actions-runner/_work/llvm-project/llvm-project/build/./bin/fakeflang  -L"/home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/24/lib/x86_64-unknown-linux-gnu" /home/gha/actions-runner/_work/llvm-project/llvm-project/flang-rt/test/Driver/endian.f90 -o /home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/flang-rt/test/Driver/Output/endian.f90.tmp
# executed command: /home/gha/actions-runner/_work/llvm-project/llvm-project/build/./bin/fakeflang -L/home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/clang/24/lib/x86_64-unknown-linux-gnu /home/gha/actions-runner/_work/llvm-project/llvm-project/flang-rt/test/Driver/endian.f90 -o /home/gha/actions-runner/_work/llvm-project/llvm-project/build/runtimes/runtimes-bins/flang-rt/test/Driver/Output/endian.f90.tmp
# .---command stderr------------
# | error: Semantic errors in /home/gha/actions-runner/_work/llvm-project/llvm-project/flang-rt/test/Driver/endian.f90
# | /home/gha/actions-runner/_work/llvm-project/llvm-project/flang-rt/test/Driver/endian.f90:97:3: error: REAL(KIND=16) is not an enabled type for this target
# |     real(16) f128(2)
# |     ^^^^^^^^^^^^^^^^
# `-----------------------------
# error: command failed with exit status: 1

--

```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/218302