Always build GHCi libs

Since the introduction of -split-sections, using GHCi with the RTS
linker is really slow:

```
$ time (echo :quit | ./inplace/bin/ghc-stage2 --interactive -fexternal-interpreter)
GHCi, version 8.1.20170304: http://www.haskell.org/ghc/  :? for help
Prelude> Leaving GHCi.

real        0m3.793s
```

(when we use `-fexternal-interpreter` it uses the RTS linker by default,
you can make it use the system linker by adding `-dynamic`)

Building the GHCi libs doesn't take much time or space in the GHC build,
but makes things much quicker for people using the RTS linker:

```
$ time (echo :quit | ./inplace/bin/ghc-stage2 --interactive -fexternal-interpreter)
GHCi, version 8.1.20170304: http://www.haskell.org/ghc/  :? for help
Prelude> Leaving GHCi.

real        0m0.285s
```

So I propose that we build and ship them unconditionally.

Test Plan: validate, perf tests above

Reviewers: bgamari, austin, niteria, erikd, Phyx

Reviewed By: bgamari

Subscribers: rwbarton, thomie, snowleopard

Differential Revision: https://phabricator.haskell.org/D3298

rules/build-package-data.mk

@@ -32,11 +32,13 @@ endif
 $1_$2_CONFIGURE_OPTS += --disable-library-for-ghci
 ifeq "$$(filter v,$$($1_$2_WAYS))" "v"
 $1_$2_CONFIGURE_OPTS += --enable-library-vanilla
+# Build the GHCi lib even if GHCi is dynamic (and therefore won't use
+# these by default), because they will be used by
+#  (a) ghci -fexternal-interpreter
+#  (b) statically-linked binaries that use the GHC package
 ifeq "$$(GhcWithInterpreter)" "YES"
-ifneq "$$(DYNAMIC_GHC_PROGRAMS)" "YES"
 $1_$2_CONFIGURE_OPTS += --enable-library-for-ghci
 endif
-endif
 else
 $1_$2_CONFIGURE_OPTS += --disable-library-vanilla
 endif

rules/build-package-way.mk

@@ -143,9 +143,6 @@ endif
 endif
 
 # Build the GHCi library
-ifeq "$$(DYNAMIC_GHC_PROGRAMS)" "YES"
-$1_$2_GHCI_LIB = $$($1_$2_dyn_LIB)
-else
 ifeq "$3" "v"
 $1_$2_GHCI_LIB = $1/$2/build/HS$$($1_$2_COMPONENT_ID).$$($3_osuf)
 ifeq "$$($1_$2_BUILD_GHCI_LIB)" "YES"
@@ -164,7 +161,6 @@ $(call all-target,$1_$2,$$($1_$2_GHCI_LIB))
 endif
 endif # "$$($1_$2_BUILD_GHCI_LIB)" "YES"
 endif # "$3" "v"
-endif # "$$(DYNAMIC_GHC_PROGRAMS)" "YES"
 
 $(call profEnd, build-package-way($1,$2,$3))
 endef # build-package-way