RFR: 8295070: Introduce more target combinations for compiler flags

[Julian Waters]

Several parts of the make system in the JDK has large parts of cluttered if branches dedicated to setting flags for the specific compiler used in the build. This could be more neatly accomplished by instead adding more target combinations in SetupNativeCompilation so the callsite can more cleanly specify which OS and compiler it desires to set these flags for. The change currently includes:

DISABLED_WARNINGS (including per file warnings), CFLAGS/CXXFLAGS and LDFLAGS: Compiler-then-OS (The priority shown reflects how the flags depend more so on the compiler)
LIBS: OS-then-Compiler, as libraries typically vary based on the system being compiled for

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Commit messages:
 - Document changes
 - Extra combinations for CFLAGS, DISABLED_WARNINGS, LDFLAGS and LIBS

Changes: https://git.openjdk.org/jdk/pull/10634/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10634&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295070
  Stats: 47 lines in 1 file changed: 37 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/10634.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10634/head:pull/10634

PR: https://git.openjdk.org/jdk/pull/10634