Build with yasm on Win64 as well.

Chromium's doesn't have built-in support for ml64.exe. Seems easier to
just build consistently with Yasm on both Win32 and Win64. (This will
require an equivalent change in Chromium's build, but keep upstream
and downstream builds consistent.)

Also don't set CMAKE_ASM_NASM_COMPILER explicitly; cmake's default
ASM_NASM behavior will search for both nasm or yasm in %PATH%. Leave
it unset so it can be overwritten on the command-line to point to
a particular yasm. Update BUILDING accordingly.

Verified the tests still pass.

Change-Id: I7e434be474b5b2d49e3bafbced5b41cc0246bd00
Reviewed-on: https://boringssl-review.googlesource.com/2104
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/BUILDING b/BUILDING
index ee6b0ab..cf239e9 100644
--- a/BUILDING
+++ b/BUILDING
@@ -25,4 +25,9 @@
 BORINGSSL_SHARED_LIBRARY defined in the code which #includes the BoringSSL
 headers.
 
+To build on Windows, Yasm[2] is required for assembly. Either ensure yasm.exe
+is in %PATH% or configure CMAKE_ASM_NASM_COMPILER appropriately. Note that
+full Windows support is still in progress.
+
 [1] http://martine.github.io/ninja/
+[2] http://yasm.tortall.net/