RFR: 8352284: EXTRA_CFLAGS incorrectly applied to BUILD_LIBJVM src/hotspot C++ source files [v3]

[Patrick Zhang]

> Building jdk with `--with-extra-cflags='-Wno-incompatible-pointer-types'` triggers 1000+ warning messages like `cc1plus: warning: command-line option ‘-Wno-incompatible-pointer-types’ is valid for C/ObjC but not for C++`. 
> 
> The root cause is that `JVM_CFLAGS ` which contains both `EXTRA_CXXFLAGS` and `EXTRA_CFLAGS` when compiling `src/hotspot` C++ source files and building `BUILD_LIBJVM`.
> 
> This PR does:
> 1. Not to append `EXTRA_CFLAGS` or `EXTRA_CXXFLAGS` into `JVM_CFLAGS` before calling `SetupJdkLibrary`, instead let `SetupCompilerFlags` accept and merge `EXTRA_CFLAGS` and `EXTRA_CXXFLAGS` passed from `SetupJdkLibrary` as parameters, so CPP compilation will only see `EXTRA_CXXFLAGS` as expected.
> 2. Correct `PCH_COMMAND` to use `EXTRA_CXXFLAGS` as precompiled.hpp.gch should not be compiled with `EXTRA_CFLAGS`.
> 3. Fixed `STATIC_LIB_CFLAGS` in `Flags.gmk` to `-DSTATIC_BUILD=1`, which was missed by [cbab40bc](https://github.com/openjdk/jdk/commit/cbab40bce45a2f58906be49c841178fa1dfd457e#diff-ab3ce05e795360030f19402fd0c2fad1dc1f7c5e7acc993cc4a2096cf31ccf40R114-R121) for the refactor of building static libs.
> 
> Tests: Passed jdk building on an AArch64 Linux system and tier1 sanity tests, also passed OpenJDK GHA Sanity Checks.

Patrick Zhang has updated the pull request incrementally with one additional commit since the last revision:

  Remove the comments wrt not appending EXTRA_CFLAGS to JVM_CFLAGS
  
  Signed-off-by: Patrick Zhang <patrick at os.amperecomputing.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24115/files
  - new: https://git.openjdk.org/jdk/pull/24115/files/aa242dd5..5440ddc5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24115&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24115&range=01-02

  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24115.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24115/head:pull/24115

PR: https://git.openjdk.org/jdk/pull/24115