Proposal: Add -buildmode parameter support

[cpunion]

Background

The Go compiler supports multiple build modes via the -buildmode parameter. Currently, llgo only supports building standard executables. This proposal adds support for library generation modes needed for issue #1194.

Proposed Build Modes

Support only the essential modes:

• exe - Standard executable (default, already supported)
• c-archive - C archive file (.a + .h header)
• c-shared - C shared library (.so/.dll/.dylib)

Implementation

1. Command Line Interface

llgo build [-buildmode mode] [-o output] [packages]

2. Code Changes

• Add BuildMode flag to cmd/internal/flags/flags.go
• Extend Config struct in internal/build/build.go
• Modify linker arguments based on build mode
• Generate C headers for c-archive and c-shared modes

Use Case for Issue #1194

The c-archive mode will generate:

• Static library (.a file) containing Go code
• C header file (.h) with exported function declarations

This enables Go code to be compiled as libraries for target platforms rather than executables, which is required by issue #1194.