gcc: remove aligned_alloc hack on Darwin

In a similar vein as PR NixOS#226048 this hack doesn't seem necessary anymore
and introduces build differences across linux/darwin.

pkgs/development/compilers/gcc/common/pre-configure.nix

@@ -49,38 +49,6 @@ in lib.optionalString (hostPlatform.isSunOS && hostPlatform.is64bit) ''
   export AS_FOR_TARGET=${gnatboot}/bin/gas
 ''
 
-# NOTE 2020/3/18: This environment variable prevents configure scripts from
-# detecting the presence of aligned_alloc on Darwin.  There are many facts that
-# collectively make this fix necessary:
-#  - Nix uses a fixed set of standard library headers on all MacOS systems,
-#    regardless of their actual version.  (Nix uses version 10.12 headers.)
-#  - Nix uses the native standard library binaries for the build system.  That
-#    means the standard library binaries may not exactly match the standard
-#    library headers.
-#  - The aligned_alloc procedure is present in MacOS 10.15 (Catalina), but not
-#    in earlier versions.  Therefore on Catalina systems, aligned_alloc is
-#    linkable (i.e. present in the binary libraries) but not present in the
-#    headers.
-#  - Configure scripts detect a procedure's existence by checking whether it is
-#    linkable.  They do not check whether it is present in the headers.
-#  - GCC throws an error during compilation because aligned_alloc is not
-#    defined in the headers---even though the linker can see it.
-#
-# This fix would not be necessary if ANY of the above were false:
-#  - If Nix used native headers for each different MacOS version, aligned_alloc
-#    would be in the headers on Catalina.
-#  - If Nix used the same libary binaries for each MacOS version, aligned_alloc
-#    would not be in the library binaries.
-#  - If Catalina did not include aligned_alloc, this wouldn't be a problem.
-#  - If the configure scripts looked for header presence as well as
-#    linkability, they would see that aligned_alloc is missing.
-#  - If GCC allowed implicit declaration of symbols, it would not fail during
-#    compilation even if the configure scripts did not check header presence.
-#
-+ lib.optionalString (hostPlatform.isDarwin) ''
-  export ac_cv_func_aligned_alloc=no
-''
-
 # In order to properly install libgccjit on macOS Catalina, strip(1)
 # upon installation must not remove external symbols, otherwise the
 # install step errors with "symbols referenced by indirect symbol