[lldb] skip host build for lldb_tblgen with LLDB_TABLEGEN_EXE set

When cross compiling lldb-server, do not create a host build
for building lldb-tblgeb when LLDB_TABLEGEN_EXE is already
provided. This avoids an expensive and time-consuming build step
if lldb-tblgen was already built previously for host.

Reviewed By: JDevlieghere

Differential Revision: https://reviews.llvm.org/D108053