Loader for x64 application ?

[sendersu]

have a look over asmjit and/or blackbone libs

[mudlord]

Yes, use Xbyak or AsmJIT to build up the assembler/shellcode/code fragments, and then use Blackbone for the actual work. You could use something like mhook to make hook functions though and use NASM for pure 64bit assembler functions.

Blackbone is quite extensive and should be more than enough for your needs.
It assumes you know C++ though.
Same goes for Xbyak and AsmJit.