This change adds comprehensive TLS General Dynamic model support across
all architectures to enable dlopen() of Go shared libraries on
non-glibc systems.
Implementation includes:
- R_*_TLS_GD relocation support for all architectures (amd64, 386,
 arm64, arm, ppc64, s390x, riscv64, loong64, mips/mips64)
- TLS_GD instruction sequences in assemblers for proper __tls_get_addr
 calling convention
- Linker support for TLS GD relocations with external linking
- Automatic GD model selection for c-shared/c-archive builds on Unix
- Preserved compatibility with existing glibc IE model behavior
Fixes dlopen() incompatibility caused by glibc-specific TLS behavior
that prevents Go shared libraries from loading on other dynamic
loaders and libc implementations.
Co-Authored-By: Alexander Musman <alexander.musman@gmail.com>

This change extends null-safety checks to handle argc/argv absence
across all Unix platforms for shared library builds.
When running as c-archive or c-shared on non-glibc systems, argv may
be nil since DT_INIT_ARRAY constructors don't receive arguments per
ELF specification. Only glibc provides argc/argv as a non-standard
extension.
Added universal null checks in sysargs() for:
- Linux: Handles standards-compliant libc implementations
- Darwin/macOS: Prevents crashes when using c-shared builds
- FreeBSD/NetBSD/OpenBSD/DragonFly: BSD libc follows ELF spec strictly
- Solaris: Standards-compliant libc implementation
Prevents SIGSEGV crashes while maintaining full backward compatibility
with glibc systems.

This change adds comprehensive documentation and testing infrastructure
for the non-glibc compatibility fixes.
Documentation includes:
- Technical details of TLS General Dynamic implementation
- argc/argv SIGSEGV fix explanation with ELF specification references
- Proper attribution to prior work by Alexander Musman and related
 GitHub issues (golang#71953, golang#73667)
- Standards references for ELF gABI and TLS specifications
Testing infrastructure includes:
- Docker-based multi-architecture test framework
- Alpine Linux (musl) and Ubuntu (glibc) test containers
- Comprehensive c-shared library dlopen() validation
- Test cases for both TLS GD and argc/argv fixes
All documentation uses plain text format following Go project
requirements with 76-character line wrapping.

Add -tls flag to control Thread Local Storage model selection with
options: auto, LE (Local Exec), IE (Initial Exec), GD (General Dynamic).
Default behavior selects GD for c-shared/c-archive builds on Unix
platforms to enable dlopen() compatibility with non-glibc systems.
Validates TLS model selection to prevent invalid combinations such as
LE with shared libraries. Updates all architectures (x86, ARM64, ARM,
PowerPC64, s390x, RISC-V, LoongArch64, MIPS) to use centralized TLS
model selection logic through ShouldUseTLSGD() and ShouldUseTLSLE()
helper functions.
Also fixes missing variable declaration in runtime/os_linux.go that
was causing build failures.
Fixes golang#13492
Co-Authored-By: Alexander Musman <alexander.musman@gmail.com>