[RISC-V] Correct RiscV64Context structure size field

[viewizard]

Correct RiscV64Context structure size field according to runtime's registers context structure size with alignment. This commit fix parallelstacks SOS command in diagnostics:

before:

> parallelstacks

==> 0 threads with 0 roots

after

> parallelstacks

________________________________________________
 ~~~~ 1d8f51
    1 Interop+Sys.ReadStdin(Byte*, Int32)
    1 System.IO.StdInReader.ReadStdin(Byte*, Int32)
    1 System.IO.StdInReader.ReadKey()
    1 System.IO.StdInReader.ReadLineCore(Boolean)
    1 System.IO.StdInReader.ReadLine()
    1 System.IO.SyncTextReader.ReadLine()
    1 System.Console.ReadLine()
    1 TestApp.Program.Main()
    1 TestApp.Program.Main()

==> 1 threads with 1 roots

CC @clamp03 @wscho77 @HJLeee @JongHeonChoi @t-mustafin @gbalykov

[gbalykov]

@leculver please take a look

[leculver]

Sorry for the delay, I am just back from vacation.

[gbalykov]

@leculver thank you!