gollvm: pass -fPIC to libgmp/mpfr/mpc builds

If we are building PIC, pass -fPIC to external library builds.
Otherwise linking step may fail with relocation error.

Change-Id: I639713819d7706ca5bf028f9172d3a13316a8cb1
Reviewed-on: https://go-review.googlesource.com/127955
Reviewed-by: Than McIntosh <thanm@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5674f75..86109ec 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -29,6 +29,9 @@
 if(NOT "${CMAKE_SYSROOT}" STREQUAL "")
   string(APPEND EXTCPPFLAGS " --sysroot=${CMAKE_SYSROOT}")
 endif()
+if(LLVM_ENABLE_PIC)
+  string(APPEND EXTCPPFLAGS " -fPIC")
+endif()
 set(EXTLDFLAGS "LDFLAGS=-L${EXTLIBDIR}")
 set(EXTCC "CC=${CMAKE_C_COMPILER}" "CXX=${CMAKE_CXX_COMPILER}")