RFR: JEP 386 patch - minor cleanup

[Alexander Scherbatiy]

"JEP 386 patch - minor cleanup" changes discussed on portola-dev alias:
https://mail.openjdk.java.net/pipermail/portola-dev/2020-August/000471.html

Changed:
* RELEASE_FILE_OS_LIBC is introduced in make files
* glibc_version is renamed to libc_version
* added fix for numa_node_to_cpus NUMA method
* WhiteBox is used to check musl libc in tests
* Platform.isBusybox(tool) method is added to check busybox in tests

Removed:
* HOTSPOT_TARGET_LIBC flag
* workaround for issues for execvp file call in childproc.c file
* workaround for coredump issue in ps_core.c file

Skipped:
* isnanf change from globalDefinitions_gcc.hpp
* SIGRTMAX changes from NativeThread.c and linux_close.c files

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

Commit messages:
 - JEP 386 patch - Do not workaround code for coredump. Fix it as a separate issue.
 - JEP 386 patch - remove unused imports from VMProps.java
 - JEP 386 patch - fix identation in libc_version declaration
 - JEP 386 patch - introduce RELEASE_FILE_LIBC
 - JEP 386 patch - fix identantion in TestInheritFD.java test
 - JEP 386 patch - remove isMuslLibc() method from TestHelper.java (it is replaced by WhiteBox call)
 - JEP 386 patch - remove execvp() workaround from childproc.c
 - JEP 386 patch - remove smallest_ptr initialization from genCollectedHeap.cpp
 - JEP 386 patch - remove INCLUDE_SA_ATTACH flag
 - JEP 386 patch - remove OPENJDK_TARGET_LIBC and stringop-truncation flags from make files
 - ... and 17 more: https://git.openjdk.java.net/portola/compare/3010a3d4...4d1aacd5

Changes: https://git.openjdk.java.net/portola/pull/12/files
 Webrev: https://webrevs.openjdk.java.net/portola/12/webrev.00
  Stats: 392 lines in 35 files changed: 137 ins; 135 del; 120 mod
  Patch: https://git.openjdk.java.net/portola/pull/12.diff
  Fetch: git fetch https://git.openjdk.java.net/portola pull/12/head:pull/12

PR: https://git.openjdk.java.net/portola/pull/12