Remove support for GCC 4.7.

This reverts https://boringssl-review.googlesource.com/24924. As noted
there, GCC 4.7 support ends 2018-03-23, which has passed. GCC 4.8.0 was
released 2013-03-22, so we are now past the five year mark, matching
Abseil's guidelines.

Abseil also now explicitly lists supported compilers and explicitly
requires GCC 4.8+. https://abseil.io/docs/cpp/platforms/platforms

gRPC also now requires 4.8 per
https://github.com/grpc/grpc/issues/10036#issuecomment-290248204

Update-Note: On the off chance someone was using GCC 4.7, which only
started working in January, that'll no longer work.

Change-Id: Ie017822e903f98293e7b5e9bda10f104f17be7b3
Reviewed-on: https://boringssl-review.googlesource.com/c/32564
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

crypto/internal.h

@@ -117,14 +117,7 @@
 #include <string.h>
 
 #if !defined(__cplusplus)
-#if defined(__GNUC__) && \
-    (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__) < 40800
-// |alignas| and |alignof| were added in C11. GCC added support in version 4.8.
-// Testing for __STDC_VERSION__/__cplusplus doesn't work because 4.7 already
-// reports support for C11.
-#define alignas(x) __attribute__ ((aligned (x)))
-#define alignof(x) __alignof__ (x)
-#elif defined(_MSC_VER)
+#if defined(_MSC_VER)
 #define alignas(x) __declspec(align(x))
 #define alignof __alignof
 #else