Use thumb instruction set for ldb-server on android arm

[tberghammer]

Summary

Use thumb instruction set for ldb-server on android arm

This change reduce the size of the lldb-server binary on android-arm from a release build by ~5MB

[labath]

lgtm

[ovyalov]

LGTM

[omjavaid]

-mthumb will force T32 instruction set while -marm will force A32.

Best is not to use any of these flags to let the compiler decide best
possible instruction set combination.

[tberghammer]

In D14816#292929, @omjavaid wrote:

-mthumb will force T32 instruction set while -marm will force A32.

Best is not to use any of these flags to let the compiler decide best
possible instruction set combination.

I think because we specify -march=armv7-a to have the necessary 64 bit atomic instructions the compiler decide to use arm in every case. Forcing the instruction set to thumb reduces the size of the final release binary by ~30% while have negligible run time speed impact so it will help us speed up the iteration cycle (smaller lldb-server means we have to copy less data between host and target).

lldb/trunk/cmake/platforms/Android.cmake

 
 set( ANDROID_CXX_FLAGS "--sysroot=${ANDROID_SYSROOT} -funwind-tables -fsigned-char -no-canonical-prefixes" )
 # TODO: different ARM abi have different flags such as neon, vfpv etc
 if( X86 )
  set( ANDROID_CXX_FLAGS "${ANDROID_CXX_FLAGS} -funswitch-loops -finline-limit=300" )
 elseif( ANDROID_ABI STREQUAL "armeabi" )
  # 64 bit atomic operations used in c++ libraries require armv7-a instructions
  # armv5te and armv6 were tried but do not work.
- set( ANDROID_CXX_FLAGS "${ANDROID_CXX_FLAGS} -march=armv7-a" )
+ set( ANDROID_CXX_FLAGS "${ANDROID_CXX_FLAGS} -march=armv7-a -mthumb" )
  if( LLVM_BUILD_STATIC )
   # Temporary workaround for static linking with the latest API.
   set( ANDROID_CXX_FLAGS "${ANDROID_CXX_FLAGS} -DANDROID_ARM_BUILD_STATIC" )
  endif()
 elseif( ANDROID_ABI STREQUAL "mips" )
  # http://b.android.com/182094
  list( FIND LLDB_SYSTEM_LIBS atomic index )
  if( index EQUAL -1 )