runtime: fix addmoduledata to follow the platform ABI

addmoduledata is called from a .init_array function and need to follow the
platform ABI. It contains accesses to global data which are rewritten to use
R15 by the assembler, and as R15 is callee-save we need to save it.

Change-Id: I03893efb1576aed4f102f2465421f256f3bb0f30
Reviewed-on: https://go-review.googlesource.com/9941Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testshared/test.bash

@@ -78,6 +78,7 @@ ensure_ldd $rootdir/libdep.so $std_install_dir/$soname
 
 # And exe that links against both
 go install -installsuffix="$mysuffix" -linkshared exe
+./bin/exe || die "./bin/exe failed with code $?"
 ensure_ldd ./bin/exe $rootdir/libdep.so
 ensure_ldd ./bin/exe $std_install_dir/$soname
 

src/runtime/asm_amd64.s

@@ -1693,8 +1693,10 @@ TEXT runtime·prefetchnta(SB),NOSPLIT,$0-8
 	RET
 
 // This is called from .init_array and follows the platform, not Go, ABI.
-TEXT runtime·addmoduledata(SB),NOSPLIT,$0-8
+TEXT runtime·addmoduledata(SB),NOSPLIT,$0-0
+	PUSHQ	R15 // The access to global variables below implicitly uses R15, which is callee-save
 	MOVQ	runtime·lastmoduledatap(SB), AX
 	MOVQ	DI, moduledata_next(AX)
 	MOVQ	DI, runtime·lastmoduledatap(SB)
+	POPQ	R15
 	RET