Rival platform crashes with Rival3

[JonasRegehr]

racket -y src/main.rkt report --platform rival bench/tutorial.fpcore out

Crashes with this error on main:

expr->ffi: Unknown expression: (lgamma x)
  context...:
   .../private/map.rkt:40:19: loop
   [repeats 1 more time]
   /Users/jonasregehr/Library/Racket/8.17/pkgs/rival3/main.rkt:392:0: rival-compile
   /Users/jonasregehr/herbie/src/syntax/rival.rkt:151:0: make-real-compiler
   /Users/jonasregehr/herbie/src/syntax/generators.rkt:24:15
   /Users/jonasregehr/herbie/src/syntax/platform-language.rkt:72:0: check-fl-proc!
   /Users/jonasregehr/herbie/src/syntax/platform-language.rkt:102:0
   .../syntax/platform-language.rkt:246:7
   body of "/Users/jonasregehr/herbie/src/platforms/rival.rkt"
   /Applications/Racket v8.17/collects/racket/private/more-scheme.rkt:377:2: hash-ref!
   /Users/jonasregehr/herbie/src/syntax/load-platform.rkt:34:0: activate-platform!
   /Users/jonasregehr/herbie/src/api/run.rkt:42:0: make-report
   body of (submod "/Users/jonasregehr/herbie/src/main.rkt" main)

It looks like Rival3 doesn't support lgamma and tgamma. We should either remove these from the Rival platform or add them to Rival3.