Add LINUX_GNU_TOOLCHAIN=1 in make command to use riscv64-linux-gnu-
toolchain

Merge branch 'southlake' into master

MMIO llcop takes time to take effect. Delay needs to be added before
checking llcop result.

Only 64 writes now.

* to test (mcycle / mtime) == 100

* note that --no-diff is require to do the test as INVALIDATE op will break coherence

Known problem: extintr test may stuck

CPUSS Peripheral, 0x1f_0000_0000, 0x1f_0fff_ffff

Detailed address map:

device, address_begin, address_end
CLINT, 0x1f_0000_0000, 0x1f_0000_ffff
BEU, 0x1f_0001_0000, 0x1f_0001_ffff
Debug Module, 0x1f_0002_0000, 0x1f_0002_0fff
MMPMA, 0x1f_0003_0000, 0x1f_0003_0fff
L3CacheCtrl, 0x1f_0004_0000, 0x1f_0004_1fff
reserved
PLIC, 0x1f_0c00_0000, 0x1f_0fff_ffff

Address map in tests to be updated