commonlib/helpers: Add alloca() macro

If there is no alloca() macro defined, then define it as
__builtin_alloca(), as most compilers support that as an intrinsic
instead, if no alloca.h is included.

Change-Id: I8730d57ce7c2f323b52e7a5720c598fb7af16b44
Signed-off-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/36387
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/commonlib/include/commonlib/helpers.h

@@ -137,4 +137,8 @@
 #define __unused __attribute__((unused))
 #endif
 
+#ifndef alloca
+#define alloca(x) __builtin_alloca(x)
+#endif
+
 #endif /* COMMONLIB_HELPERS_H */