Append to CFLAGS_EXTRAS and LD_EXTRAS when building cmdline.

When building the command-line for compilations during tests, append to
CFLAGS_EXTRAS and LD_EXTRAS to preserve switches set by the environment
(i.e. for cross compiling to test on another platform).

Test Plan:
TestCPP11EnumTypes.py passes testing from macosx -> linux remotely with cross
compiling args in CFLAGS_EXTRAS and LD_EXTRAS.

Differential Revision: http://reviews.llvm.org/D8942

llvm-svn: 235083