[libc] Make static_assert() available via assert.h

Expose static_assert() via assert.h and migrate link-time assertions
to build-time assertions where possible.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/assert.h

@@ -55,6 +55,15 @@ assert_printf ( const char *fmt, ... ) asm ( "printf" );
 		} 							     \
 	} while ( 0 )
 
+/**
+ * Assert a condition at build time
+ *
+ * If the compiler cannot prove that the condition is true, the build
+ * will fail with an error message.
+ */
+#undef static_assert
+#define static_assert(x) _Static_assert( x, #x )
+
 /**
  * Assert a condition at link-time.
  *