build: add -mcpu for cortex-a72 and cortex-a76

The aarch64 toolchain is built without --with-cpu, so the
CPU_CFLAGS_<type> entry in target.mk is the only place a -mcpu option
comes from. The cortex-a72 (bcm27xx/bcm2711, mvebu/cortexa72) and
cortex-a76 (bcm27xx/bcm2712) CPU types have no entry, so userspace for
these subtargets is built with plain "-Os -pipe", which produces
generic armv8-a code. The "doesn't correspond to a known type" warning
is not visible because the target scan discards stderr.

Add the missing entries. For cortex-a76 this also enables the ARMv8.2
features such as LSE atomics, which replace the out of line atomic
helper calls.

Fixes: 2534141 ("mvebu: add support for MACCHIATObin (cortex-a72)")
Fixes: 5e928ac ("brcm2708: add bcm2711 subtarget (RPi 4B aarch64)")
Fixes: 8b63d9a ("bcm27xx: add new bcm2712 subtarget")
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Link: openwrt/openwrt#25627
Signed-off-by: Robert Marko <robimarko@gmail.com>

Author: hauke

include/target.mk

@@ -265,6 +265,8 @@ ifeq ($(DUMP),1)
     CPU_TYPE ?= generic
     CPU_CFLAGS_generic = -mcpu=generic
     CPU_CFLAGS_cortex-a53 = -mcpu=cortex-a53
+    CPU_CFLAGS_cortex-a72 = -mcpu=cortex-a72
+    CPU_CFLAGS_cortex-a76 = -mcpu=cortex-a76
   endif
   ifeq ($(ARCH),riscv64)
     CPU_TYPE ?= generic