WASM_X64: Support local variables

[ubaidsk]

This PR adds support for local variables in the wasm_x64 backend.

Examples:

$ cat examples/expr2.py 
def main0():
    x: i32
    x = (2+3)*5
    y: i32
    z: i32
    y = 14
    z = 2
    print(x + y * z)

main0()
$ lpython examples/expr2.py --backend wasm_x64 -o tmp
Call to print_i32() will be printed as exit code
Call to flush_buf() is not yet supported
$ ./tmp
$ echo $?
53
$ cat examples/expr2.py 
def sqr(x: i32) -> i32:
    return x * x

def main0():
    pi: i32 = 3
    r: i32 = 5
    area: i32
    area = pi * sqr(r)
    print(area)

main0()
$ lpython examples/expr2.py --backend wasm_x64 -o tmp
Call to print_i32() will be printed as exit code
Call to flush_buf() is not yet supported
$ ./tmp
$ echo $?
75

[ubaidsk]

The bottom two commits are part of #1360. The rest of the commits are part of this PR. I will rebase this PR after merging of #1360.

[ubaidsk]

This is ready. Please possibly review and please share feedback.

[certik]

I think this looks great.