interpreter: reload libraries when new @[Link] annotations appear in REPL

[calospoimen]

Problem

In the Crystal REPL (crystal i), libraries referenced by @[Link]
annotations are not always present when the loader is first initialized.
For example, Regex / pcre2 is only added to lib_flags on the first
use of a regular expression — after the loader has already been set up
with the prelude's libraries.

This causes a Crystal::Loader::LoadError (undefined reference to 'pcre2_compile_8') when using regex in the interpreter on Windows MinGW,
and could affect other platforms or libraries in the same scenario.

Solution

• Track @loader_lib_flags to record which flags were used at loader
  initialization.
• Add update_loader to diff the current lib_flags against the tracked
  value and load any newly-appearing libraries into the running loader.
• Call update_loader from c_function before each symbol lookup, so
  new libraries are always available.

On MinGW, also populate dll_search_paths with the crystal.exe
directory and the PATH entries so that the loader can locate DLLs at
runtime.

Testing

Tested on Windows MinGW UCRT64 (Crystal 1.19.1 portable installation):

# Previously raised: undefined reference to `pcre2_compile_8'
result = "hello world" =~ /world/
puts result  # => 6