build: workaround stale caches (NFC)

`LLVM_DEFAULT_TARGET_TRIPLE` is a cached variable, which means that it
may actually be unset.  Furthermore, in standalone builds, the variable
may be fully undefined.  Apply the regular expression over the empty
string in such a case.  This should improve the state of the green
dragon bot.

Author: compnerd

lldb/tools/debugserver/source/MacOSX/CMakeLists.txt

@@ -10,7 +10,7 @@
 # CFLAGS etc explicitly. Switching on LLVM_HOST_TRIPLE is also an option,
 # but it breaks down when cross-compiling.
 
-string(REGEX MATCH "^[^-]*" LLDB_DEBUGSERVER_ARCH ${LLVM_DEFAULT_TARGET_TRIPLE})
+string(REGEX MATCH "^[^-]*" LLDB_DEBUGSERVER_ARCH "${LLVM_DEFAULT_TARGET_TRIPLE}")
 
 if("${LLDB_DEBUGSERVER_ARCH}" MATCHES ".*arm.*")
   list(APPEND SOURCES arm/DNBArchImpl.cpp arm64/DNBArchImplARM64.cpp)