[CRT][LIT] build the target_cflags for Popen properly

We recently enabled crt for powerpc in
https://reviews.llvm.org/rGb7611ad0b16769d3bf172e84fa9296158f8f1910.

And we started to see some unexpected error message when running
check-runtimes.

eg:
https://lab.llvm.org/buildbot/#/builders/57/builds/9488/steps/6/logs/stdio
line 100 - 103:

"
clang-14: error: unknown argument: '-m64 -fno-function-sections'
clang-14: error: unknown argument: '-m64 -fno-function-sections'
clang-14: error: unknown argument: '-m64 -fno-function-sections'
clang-14: error: unknown argument: '-m64 -fno-function-sections'
"

Looks like we shouldn't strip the space at the beginning,
or else the command line passed to subprocess won't work well.

Reviewed By: phosek, MaskRay

Differential Revision: https://reviews.llvm.org/D108329