ghc: Do not add a space in '-U __PIC__'

GHC previously introduced a space here. However, this can in some cases
be interpreted as "-U __PIC__" - note that in shell, the -U would still
be recognized with an argument, but the argument would be " __PIC__",
with a space in front, as opposed to the single string '__PIC__'.

In practice most tools seem to handle this OK. But the Coverity Scan
analysis tool does not: it errors on the fact that ' __PIC__' is an
invalid CPP name to undefine.

With this, it seems the Coverity analysis tool can easily analyze the
entire GHC build.

Signed-off-by: Austin Seipp <austin@well-typed.com>