The correct section declaration is:
.internal_ram (NOLOAD) :
{
. = ALIGN(4);
*(.internal_ram*)
} >SRAM
The NOLOAD section attribute is documented but speaks in terms of program loaders handling the section at load time. At first this doesn't seem to apply to bare metal images but, for that purpose, objcopy acts like a program loader and honors the flag settings in the object file, omitting the section from the raw image.