Workaround with 0xffffffff80000000 address in ELF SRAM mode

[andreytkachenko]

Please add 0xffffffff80000000 address space as alias of 0x80000000 in ELF SRAM mode.

Described here

[minux]

lgtm. In fact, we can avoid -mcmodel=medany by using address range 0xffffffff80000000 to 0x7fffffff. (I can confirm that for uint32_t X and Y, any address Y is aliased with (X << 32) | Y for any X, and the cpu doesn't support execution from uncached address space.)

[vowstar]

LGTM