build: separate Android host and target compilers

[luoqianlin]

Fix Android cross-compilation by keeping host and target compilers separate.

The Android configure helper uses the Android compiler for configure.py's
target compiler detection and for GYP target rules, while host-side tools such
as ICU and V8 generators use a native compiler.

This change:

• Keeps explicit CC_target/CXX_target values authoritative.
• Treats conventional CC/CXX values as target compiler overrides when
  toolset-specific values are not set.
• Uses native gcc/g++ (or the platform equivalent) as host defaults while
  preserving explicit CC_host/CXX_host overrides.
• Accepts additional configure options and invokes configure without shell
  command concatenation.

NDK r29 with Android API 30 and arm64 is the current validation baseline.

Refs: #65771

Validation:

• python3 -m py_compile android_configure.py
• git diff --check
• Android arm64/API 30 configure and build.

[luoqianlin]

The branch has been corrected and force-pushed after the earlier bad-base update. It is now a single commit based on current main and changes only android_configure.py (33 insertions, 5 deletions). The current PR diff no longer includes the unrelated thousands of files.