soong: use optimal FPU on Cortex-A15

* Cortex-A15 supports VFPv4 instructions

Test: make otapackage
Test: Built angler with 32 bit variant set to cortex-a15.
Test: Boot this version of the angler, ran bionic unit tests.
Change-Id: I94f00c9486ece201aa2c230308b9f90b0209dc4a

cc/config/arm_device.go

@@ -121,6 +121,7 @@ var (
 		},
 		"cortex-a15": []string{
 			"-mcpu=cortex-a15",
+			"-mfpu=neon-vfpv4",
 			// Fake an ARM compiler flag as these processors support LPAE which GCC/clang
 			// don't advertise.
 			// TODO This is a hack and we need to add it for each processor that supports LPAE until some