tests relying on <iostream> are broken on Mojave builder

It appears that something has changed in the Darwin toolchain such that <iostream> is no longer found by default:

cd "driver/recomp001/recomp001.run" && $MAKE -s --no-print-directory recomp001  
Compile failed (exit code 1) errors were:
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the command line to use the libc++ standard library instead [-Wstdlibcxx-not-found]

objcpp-hi.mm:2:9: error:  fatal error: 'iostream' file not found
#import <iostream>
        ^~~~~~~~~~
1 warning and 1 error generated.
`gcc' failed in phase `C Compiler'. (Exit code: 1)

*** unexpected failure for objcpp-hi(normal)

Compile failed (exit code 1) errors were:
warning: include path for stdlibc++ headers not found; pass '-std=libc++' on the command line to use the libc++ standard library instead [-Wstdlibcxx-not-found]

/var/folders/pb/c3dc08v12yzc536lnrnngvd40000gq/T/ghc56064_0/ghc_2.cpp:1:10: error:
     fatal error: 'iostream' file not found
#include <iostream>
         ^~~~~~~~~~
1 warning and 1 error generated.
`gcc' failed in phase `C Compiler'. (Exit code: 1)

*** unexpected failure for T13366(normal)

Marking as broken.