Use configured LD for linking enc and ext libraries

"AR" was well propagated to the enc.mk and mkmf, but "LD" was not. This
caused the dynamic libraries to be linked with a linker found in the PATH,
which could be different from the one used in the Ruby build process.
This is especially important for cross-compilation, where the host
linker may not be compatible with the target system. (e.g. WebAssembly)

enc/Makefile.in

@@ -40,6 +40,7 @@ BUILTRUBY = $(topdir)/miniruby$(EXEEXT)
 
 empty =
 AR = @AR@
+LD = @LD@
 CC = @CC@
 ARFLAGS = @ARFLAGS@$(empty)
 RANLIB = @RANLIB@

lib/mkmf.rb

@@ -2149,6 +2149,7 @@ def configuration(srcdir)
 LDSHARED = #{CONFIG['LDSHARED']}
 LDSHAREDXX = #{config_string('LDSHAREDXX') || '$(LDSHARED)'}
 AR = #{CONFIG['AR']}
+LD = #{CONFIG['LD']}
 EXEEXT = #{CONFIG['EXEEXT']}
 
 }