Add 64-bit Mac OSX support

Build fails on Mac OS X, `make` output follows.

``` mf
-- Generating done
-- Build files have been written to: /Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/build
[ 0%] Generating ../event_strings.h
[ 0%] Built target generate_syslog
[ 0%] Generating include/dr_app.h, include/dr_api.h
[ 0%] Built target api_headers
Scanning dependencies of target drhelper
[ 1%] Building ASM_NASM object core/CMakeFiles/drhelper.dir/arch/asm_shared.asm.o
[ 1%] Building ASM_NASM object core/CMakeFiles/drhelper.dir/arch/x86/x86_shared.asm.o
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:207: error: symbol `mach_dep_syscall_ready' undefined
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:210: error: symbol `mach_dep_syscall_ready' undefined
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:213: error: symbol `mach_dep_syscall_ready' undefined
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:216: error: symbol `mach_dep_syscall_ready' undefined
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:284: error: symbol `dynamorio_mach_syscall_ready' undefined
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:287: error: symbol `dynamorio_mach_syscall_ready' undefined
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:290: error: symbol `dynamorio_mach_syscall_ready' undefined
/Users/wtfbbqhax/Devel/DynamoRIO/dynamorio/core/arch/x86/x86_shared.asm:293: error: symbol `dynamorio_mach_syscall_ready' undefined
make[2]: *** [core/CMakeFiles/drhelper.dir/arch/x86/x86_shared.asm.o] Error 1
make[1]: *** [core/CMakeFiles/drhelper.dir/all] Error 2
```