Improve RV64/GNU port robustness, portability, and CSR handling

[akifejaz]

improve port robustness, portability, and CSR handling

• removed tx_port.h dependency from .S files
• replaced tx_timer_interrupt.c with tx_timer_interrupt.S
• made some cleanups, formatting to better readability
• removed macros (LOAD/STORE/REGBYTES)
  changed register nomenclature to use RISC-V ABI
  names (ra, sp, t0, etc.)
• added readme_threadx.txt

[akifejaz]

started writing the readme risc-v64/gnu/readme_threadx.txt for rv64/gnu, please feel free to comment what to add/remove.

[akifejaz]

Hi @billlamiework, as per your suggestions here #490 (comment), I've updated the RV64/GNU port, It should include following changes. Please review the changes when you get time.

• removed tx_port.h dependency from .S files
• replaced tx_timer_interrupt.c with tx_timer_interrupt.S
• made some cleanups, formatting to better readability
• removed macros (LOAD/STORE/REGBYTES)
• changed register nomenclature to use RISC-V ABI
  names (ra, sp, t0, etc.)
• added readme_threadx.txt

Once we have risc-v64/gnu port fixed/formatted correctly, I'll do the same for risc-v32/gnu.
cc: @fdesbiens

[billlamiework]

Definitely looking good... my only comments are:

1. It's good(better) to us inline assembly or compiler intrinsics for TX_DISABLE/TX_RESTORE macros in tx_port.h for single core ports.
2. The assembly code seems to be using "/*" comments and "//" comments for comments on the right of assembly instructions. It would be good to use "//" if possible. Also, it would be good if all these started in the same column.
   Otherwise, looking good!