RFR: 8314262: GHA: Cut down cross-compilation sysroots deeper

[Aleksey Shipilev]

[JDK-8294941](https://bugs.openjdk.org/browse/JDK-8294941) cut out some unused files in GHA sysroots. But I notice that even after that, we still have quite a bit of fluff there, especially in RISC-V sysroot. We can cut a bit deeper. Notably, we don't need runnable gcc in the sysroot, because we use the cross-compilation toolchain from the host.

This further saves bandwidth, cache size, etc. for actively developed projects.

Seeing the following improvements in sysroot sizes:


 ARM: 64 MB -> 37 MB
 AArch64:  80 MB -> 46 MB
 S390X: 69 MB -> 41 MB
 PPC64: 84 MB -> 51 MB
 RISC-V: 287 MB -> 53 MB

Total: 584 MB -> 228 MB (-61%)


Additional testing:
 - [x] GHA (cross-builds still work fine)

-------------

Commit messages:
 - Correction: libexec/gcc should be removed explicitly
 - Fix

Changes: https://git.openjdk.org/jdk/pull/15287/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15287&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314262
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15287.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15287/head:pull/15287

PR: https://git.openjdk.org/jdk/pull/15287