cpu/qemu-x86: Increase heap size

On x86_64, the default heap size is too small when using 32 CPUs.

Change-Id: Ib4f770a7a54d975d213b2456cc7d1ed9151cb6f9
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55761
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

src/cpu/qemu-x86/Kconfig

@@ -50,4 +50,7 @@ config MAX_CPUS
 	default 32 if SMM_TSEG
 	default 4
 
+config HEAP_SIZE
+	default 0x8000
+
 endif