Remove some unnecessary link annotations (#11563)

Co-authored-by: Sijawusz Pur Rahnama <sija@sija.pl>
Co-authored-by: Oleh Prypin <oleh@pryp.in>

src/crystal/system/unix/lib_event2.cr

@@ -1,6 +1,8 @@
 require "c/netdb"
 
-{% if flag?(:linux) %}
+# MUSL: On musl systems, librt is empty. The entire library is already included in libc.
+# The empty library is only available for POSIX compatibility. We don't need to link it.
+{% if flag?(:linux) && !flag?(:musl) %}
   @[Link("rt")]
 {% end %}
 

src/gc/boehm.cr

@@ -2,7 +2,12 @@
   require "crystal/rw_lock"
 {% end %}
 
-{% unless flag?(:win32) %}
+# MUSL: On musl systems, libpthread is empty. The entire library is already included in libc.
+# The empty library is only available for POSIX compatibility. We don't need to link it.
+#
+# OTHERS: On other systems, we add the linker annotation here to make sure libpthread is loaded
+# before libgc which looks up symbols from libpthread.
+{% unless flag?(:win32) || flag?(:musl) %}
   @[Link("pthread")]
 {% end %}
 

src/gc/none.cr

@@ -1,9 +1,3 @@
-{% unless flag?(:win32) %}
-  @[Link("pthread")]
-  lib LibC
-  end
-{% end %}
-
 module GC
   def self.init
   end

src/math/libm.cr

@@ -1,8 +1,11 @@
+# MUSL: On musl systems, libm is empty. The entire library is already included in libc.
+# The empty library is only available for POSIX compatibility. We don't need to link it.
+#
 # Interpreter: On GNU systems, libm.so is typically a GNU ld script which adds
 # the actual library file to the load path. `Crystal::Loader` does not support
 # ld scripts yet. So we just skip that for now. The libm symbols are still
 # available in the interpreter.
-{% if (flag?(:linux) && !flag?(:interpreted)) || flag?(:bsd) %}
+{% if (flag?(:linux) && !flag?(:musl) && !flag?(:interpreted)) || flag?(:bsd) %}
   @[Link("m")]
 {% end %}