Document compiler and assembler requirements.

The minimum version is purely based on what we've patched out of the perlasm
files. I'm assuming they're accurate.

Change-Id: I5ae176cf793512125fa78f203a1314396e8a14d7
Reviewed-on: https://boringssl-review.googlesource.com/8238
Reviewed-by: Adam Langley <agl@google.com>

BUILDING.md

@@ -27,10 +27,14 @@
   * A C compiler is required. On Windows, MSVC 14 (Visual Studio 2015) or later
     with Platform SDK 8.1 or later are supported. Recent versions of GCC (4.8+)
     and Clang should work on non-Windows platforms, and maybe on Windows too.
+    To build the tests, you also need a C++ compiler with C++11 support.
 
   * [Go](https://golang.org/dl/) is required. If not found by CMake, the go
     executable may be configured explicitly by setting `GO_EXECUTABLE`.
 
+  * To build the x86 and x86\_64 assembly, your assembler must support AVX2
+    instructions. If using GNU binutils, you must have 2.22 or later.
+
 ## Building
 
 Using Ninja (note the 'N' is capitalized in the cmake invocation):