Implement per-platform compiler version selection

[imciner2]

This implements the ability to select a different compiler version for each requested platform. This can be useful when certain platforms might need a newer version of GCC to fix bugs in the toolchain (e.g., riscv), but we don't want to increase the version for working platforms.

An example use of this is:

platforms = supported_platforms()
platforms = expand_cxxstring_abis(platforms)

# Use GCC 13.2 on RISC-V to fix linking issues with atomic operations, but GCC 6 everywhere else
compiler_versions = preferred_platform_compiler(platforms, v"6")
set_preferred_compiler_version!(compiler_versions, v"13.2", p -> arch(p) == "riscv64")

build_tarballs(
    ARGS,
    name,
    version,
    sources,
    script,
    platforms,
    products,
    dependencies;
    preferred_gcc_version = compiler_versions,
    julia_compat = "1.6",
)

This will then use GCC 13 on the RISCV platforms, and GCC 6 on all other platforms.