cmd/compile: support -buildmode=c-shared on linux/mips64{,le}

cmd/compile: support -buildmode=c-shared on linux/mips64{,le}

The modification of these rules is optimization to load/store global
variables. If there are a sequence of loads/stores nearby a global
variable address, the address can only be loaded from GOT once instead
of every time.

For #43264

Change-Id: Idedaf6c81f085955371320f51bca148ffb42a2d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/348732
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>