RFR: 8348327: Incorrect march flag when building libsleef/vector_math_neon.c

[Mikael Vidstedt]

[JDK-8312425](https://bugs.openjdk.org/browse/JDK-8312425) started building the libsleef library with `$(SVE_CFLAGS)` on aarch64. That variable expands to `-march=armv8-a+sve` if the compiler supports it.

The flag should only be applied to the `vector_math_sve.c` file. Applying it to the whole library means it also gets applied to `vector_math_neon.c`, and the compiler (e.g. GCC) may select SVE instructions when generating code for it. When running the resulting JDK on a CPU which does not support SVE that will lead to an illegal instruction/`SIGILL`.

Testing:

* Manually verified that `vector_math_neon.c` does *not* get built with `-march=armv8-a+sve` and that `vector_math_sve.c` (still) does get built with that flag.
* tier1-5 (in progress)

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Commit messages:
 - 8348327: Incorrect march flag when building libsleef/vector_math_neon.c

Changes: https://git.openjdk.org/jdk/pull/23245/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23245&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348327
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23245.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23245/head:pull/23245

PR: https://git.openjdk.org/jdk/pull/23245