CMake: Remove custom CMAKE_ASM_FLAGS

Chris pointed out that ".S" assembler files hint that they should be preprocessed.
Now these files have the correct extension, we don't need to be explicit about running assembly files through the preprocessor.

Bug: b/139010488
Change-Id: I1399e884071c7fc2f169e93809a9f15204e41eff
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35211
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index d0e552e..c896579 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -278,9 +278,6 @@
 # Compile flags
 ###########################################################
 
-# Run assembly files through the C preprocessor.
-SET(CMAKE_ASM_FLAGS "-x assembler-with-cpp")
-
 # Flags for project code (non 3rd party)
 set(SWIFTSHADER_COMPILE_OPTIONS "")