Fix double slashes in make paths.

CFG_BUILD_DIR, CFG_LLVM_SRC_DIR and CFG_SRC_DIR all have trailing
slashes, by definition, so this is correct.
auto merge of #10731 : chris-morgan/rust/fix-double-slashing, r=metajack

CFG_BUILD_DIR, CFG_LLVM_SRC_DIR and CFG_SRC_DIR all have trailing
slashes, by definition, so this is correct.

(This is purely cosmetic; the doubled slash is ignored by all the tools we're using.)