[libc] [llvm] [libc][math][c23] Add rsqrtf16() function (PR #137545)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `libc-aarch64-ubuntu-fullbuild-dbg` running on `libc-aarch64-ubuntu` while building `libc,utils` at step 4 "annotate".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/71/builds/31529

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

```
Step 4 (annotate) failure: 'python ../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py ...' (failure)
...
[       OK ] LlvmLibcFAbsTest.SpecialNumbers (17 us)
Ran 1 tests.  PASS: 1  FAIL: 0
[630/1064] Running unit test libc.test.src.math.smoke.canonicalizef_test.__unit__
[==========] Running 2 tests from 1 test suite.
[ RUN      ] LlvmLibcCanonicalizeTest.SpecialNumbers
[       OK ] LlvmLibcCanonicalizeTest.SpecialNumbers (6 us)
[ RUN      ] LlvmLibcCanonicalizeTest.RegularNubmers
[       OK ] LlvmLibcCanonicalizeTest.RegularNubmers (3 us)
Ran 2 tests.  PASS: 2  FAIL: 0
[631/1064] Running unit test libc.test.src.math.smoke.rsqrtf16_test.__unit__
FAILED: libc/test/src/math/smoke/CMakeFiles/libc.test.src.math.smoke.rsqrtf16_test.__unit__ 
cd /home/libc-buildbot/libc-aarch64-ubuntu/libc-aarch64-ubuntu-fullbuild-dbg/build/libc/test/src/math/smoke && /home/libc-buildbot/libc-aarch64-ubuntu/libc-aarch64-ubuntu-fullbuild-dbg/build/libc/test/src/math/smoke/libc.test.src.math.smoke.rsqrtf16_test.__unit__.__build__
[==========] Running 1 test from 1 test suite.
[ RUN      ] LlvmLibcRsqrtf16Test.SpecialNumbers
/home/libc-buildbot/libc-aarch64-ubuntu/libc-aarch64-ubuntu-fullbuild-dbg/llvm-project/libc/test/src/math/smoke/rsqrtf16_test.cpp:19: FAILURE
      Expected: __llvm_libc_20_0_0_git::fputil::test_except( static_cast<int>((0x1 | 0x2 | 0x4 | 0x8 | 0x10))) & ((0x4) ? (0x4) : static_cast<int>((0x1 | 0x2 | 0x4 | 0x8 | 0x10)))
      Which is: 0
To be equal to: (0x4)
      Which is: 4
[  FAILED  ] LlvmLibcRsqrtf16Test.SpecialNumbers
Ran 1 tests.  PASS: 0  FAIL: 1
[632/1064] Running unit test libc.test.src.math.smoke.sinf_test.__unit__.__NO_ROUND_OPT
[==========] Running 1 test from 1 test suite.
[ RUN      ] LlvmLibcSinfTest.SpecialNumbers
[       OK ] LlvmLibcSinfTest.SpecialNumbers (7 us)
Ran 1 tests.  PASS: 1  FAIL: 0
[633/1064] Running unit test libc.test.src.math.smoke.bf16divf128_test.__unit__
[==========] Running 5 tests from 1 test suite.
[ RUN      ] LlvmLibcDivTest.SpecialNumbers
[       OK ] LlvmLibcDivTest.SpecialNumbers (14 us)
[ RUN      ] LlvmLibcDivTest.DivisionByZero
[       OK ] LlvmLibcDivTest.DivisionByZero (3 us)
[ RUN      ] LlvmLibcDivTest.InvalidOperations
[       OK ] LlvmLibcDivTest.InvalidOperations (13 us)
[ RUN      ] LlvmLibcDivTest.RangeErrors
[       OK ] LlvmLibcDivTest.RangeErrors (30 us)
[ RUN      ] LlvmLibcDivTest.InexactResults
[       OK ] LlvmLibcDivTest.InexactResults (2 us)
Ran 5 tests.  PASS: 5  FAIL: 0
[634/1064] Running unit test libc.test.src.math.smoke.fminf16_test.__unit__.__NO_MISC_MATH_BASIC_OPS_OPT
[==========] Running 5 tests from 1 test suite.
[ RUN      ] LlvmLibcFMinTest.NaN
[       OK ] LlvmLibcFMinTest.NaN (4 us)
[ RUN      ] LlvmLibcFMinTest.InfArg
[       OK ] LlvmLibcFMinTest.InfArg (2 us)
[ RUN      ] LlvmLibcFMinTest.NegInfArg
[       OK ] LlvmLibcFMinTest.NegInfArg (2 us)
[ RUN      ] LlvmLibcFMinTest.BothZero
[       OK ] LlvmLibcFMinTest.BothZero (2 us)

```

</details>

https://github.com/llvm/llvm-project/pull/137545