Implement the `AT` function in linker scripts

[vishruth-thimmaiah]

The AT function is used in linker scripts to set the LMA(load address).

There are 3 primary ways of using this function:

• AT(expression) in an output section: feat: implements support for the AT keyword in output sections #2043

  .text : AT(0x1000) {*(.text)}

• AT>region Uses the next free space in a memory region.

  .data :
  {
      *(.data)
  } >RAM AT>ROM

• The PHDRS command may define a segment with the AT function.

  PHDRS {
      text PT_LOAD AT(...)
  }

The LOADADDR function should also be updated to support using the LMA address, if set. (related issue: #1769)