Config: make linker flags a string (vs a list), to be consistent with other flags

See also this email thread, "[petsc-maint] Extra linker flags on Mac", where the user had --CC_LINKER_FLAGS=-fsanitize=address --CXX_LINKER_FLAGS=-fsanitize=address on a Mac, and petsc configure failed on

def checkLinkerMac(self):
  ...
  val.append(testFlag)

since val was a string.

@sbalay fixed this in !2777 (merged) by converting string to list. But we have other flags (CFLAGS, LDFLAGS etc) work as a string, and many places in the code assume flags are strings, so in this MR I revert it back (i.e., user input was internally converted to string). Users are still allowed to do either --CC_LINKER_FLAGS=[flag1] or --CC_LINKER_FLAGS=flag1