More small fixes for icc.

Add an option to cflags for icc.
change use of asm to __asm__

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>

amd64/include/cflags.json

@@ -3,6 +3,9 @@
 		"ToolOpts": {
 			"/usr/bin/clang": [
 				"-mno-implicit-float"
+			],
+			"/opt/intel/bin/icc": [
+				"-Wmain"
 			]
 		},
 		"Cflags": [

sys/src/9/amd64/fns.h

@@ -287,7 +287,7 @@ int backtrace_list(uintptr_t pc, uintptr_t fp, uintptr_t *pcs, size_t nr_slots);
 /* horror */
 static inline void __clobber_callee_regs(void)
 {
-	asm volatile ("" : : : "rbx", "r12", "r13", "r14", "r15");
+	__asm__ volatile ("" : : : "rbx", "r12", "r13", "r14", "r15");
 }
 
 int slim_setlabel(Label*) __attribute__((returns_twice));